Imbuing is one of the most 'precious' resources in a player's arsenal. I say precious because a good player will use his imbues to fill in the gaps in his characters gear. It's important to note that imbuing is always about luck. But, as with anything else, you can carefully choose when and what to imbue, to better your odds.

I should note that I speak of Affixes in this document, without describing what they do. If you are unsure what an affix does, look them up on the Chaos Sanctuary (CS).
CS Prefix List         CS Suffix List

Some basics about imbuing:
Although noted elsewhere as +4, imbuing is based on your characterlevel (clvl) +6, for the choices of affixes on that item. That means if you're level 29, you can get affixes marked for level 35. Also note that affixes shave off 25% of their level when determining requirements, so if it's marked 40 on the CS affix sheets, it will come out as requiring 30, and 34 to get it in the imbuing process. This is why low level imbues can result in unusable items, but otherwise, you will never produce an item you can not use based on level requirements.

Imbues are luck related and sometimes you may want to decide if you're imbuing for your purposes or for trade. If you're imbuing for your purposes, you might pick items that are a bit of a gamble, but if not, go for solid items and don't go for things chancy. I'll discuss this later on.

Keep in mind when imbuing that you aren't imbuing the item you give Charsi, not really. What you are doing is giving her an item, which defines what the base item of the newly generated rare will be. The original item otherwise has no effect on the imbue. This has an affect when considering the spells on the base item: They will not carry over to the imbue.

Imbues for each class:
Each class obviously has some separate goals for imbuing, so if you're imbuing for that character, or another character, you might want to think about what 'dream items' you're going to want. For example, imbuing a helmet with a necromancer might be a bad idea, since you could get a Wormskull in the next ten minutes and toss aside your imbue. The feeling there wouldn't be too good, would it ?

Imbuing Helms and Armour:
The Affix pool for Helms and armour is shared, meaning you can get the same great affixes on helmets as you can on armour. Both have a very good pool. Here's some basics (the number given here are converted for when you can get them earliest, that would be, YOUR level).

Low level (15 or less):
Sturdy, Strong, Glorious, Lizard's, Crimson, Burgundy, Garnet, Azure, Lapis, Cobalt, Ocher, Tangerine, Coral, Beryl, Jade, Viridian, Jackal, Fox, Wolf, Tiger, Light, Remedy, Amelioration, Balance, Stability, Health, Warding, Thorns, Spikes and Ease.

Medium level (16-30):
Blessed, Saintly, Holy, Amber, Mammoth, Colossus, Life, Defiance, Simplicity

High Level (31+):
Serpent's (31), Dragon's (44), Skill (39), Precision (54), Measure (31), Regeneration (66),

As you can tell, there isn't a lot of 'bad' affixes in the Armour sections. Light, Life, Thorns, Spikes and Health (which is useful against big D) are the only really 'bad' affixes if you go for imbuing before 30. If you're Barbarian, I'd cut it off at 30. For a necro or sorc, you might want to consider the 'risk' of the mana bonus, but that's up to you. A bowazon obviously wouldn't mind getting the Precision affix.

Imbuing Weapons:
(Note that Weapons includes Swords, Axes, Polearms, Spears, Mauls and Daggers) Weapons are a lot trickier than Armour imbues. Whereas armour gets better as the level goes up and then gets some not so great affixes, weapons get some extremely bad affixes or ones that you would probably NOT want. I'm not going into the same detail as I did with Armour.

Basically, you want to decide if you want to risk it all or be conservative with your imbues. At level 26, you can get Lord's, Merciless, Meteoric, Slaughter, Vampire, Locust and Quickness. Seeing as those are the seven best Affixes to get on a weapon, except for Lord's being worse than King's, you may want to take your shot at it then and there. If you're a barbarian, since you would want to get +2 skills on your weapon, things are a little riskier. Berserker's Occurs at level 34, but so does (or before then) Serpent's, Burgundy, Lapis, Tangerine and Jade. These aren't really combat based Affixes and don't add to the power of your weapon. I should note that although the CS does not list the lowest level resistance affixes occurring on weapons, and therefore before those four I spoke of 'open up' four unwanted choices. However, I'm pretty sure I have seen 6% fire resist on a weapon before, which may lead to the CS not supplying all the data I need.

Imbuing Mage Weapons:
(mage Weapons are Scepters, Wands and Staves)
My thoughts on Imbuing Mage Weapons: Don't. Honestly, the odds that you will produce anything worthwhile are so incredibly low. Keep this in mind: If you're looking for a combat weapon from items of this set, you can get mage abilities, like Fastest Cast, +mana and so on. And if you're looking for a magic user's weapon, you can get Knights', Carnage and other modifiers you'd have no use for. This is the reason good rare divine scepters, grave wands and so forth aren't that common on BNet. If you do want to imbue a Scepter, imbue it at level 29, since you can get King's then. You can't avoid the chance you'll get resists, +dex, and everything else you don't want, they are always on Scepters. So it's always a gamble, and with scepters, the odds are you won't get an item you want. (But you can, so if you feel lucky ...)

Imbuing Bows:
Bows are generally the same as Weapons; however, they have access to dex bonuses, which are desirable for Bowazons or any one else using a bow. They can also get the Sapphire prefix, as of level 19. Also, Bows can't get the Quickness or Swiftness affixes, nor can they get Knockback (damn !) or Reduce monster defense per hit. Generally they imbue much the same as weapons (26 is still a good level) with a few slight changes.

Imbuing Shields:
Shields are a bit of an enigma: Their best Affix is probably Deflecting, which is available when you get your first imbue at the lowest level possible (8). Actually, it's available before then (five, actually) but you can't get your imbue then. Shields basically follow the same rules to imbuing as Armour, except for the blocking/and deflection. If you're interested in producing a shield with Deflecting and not DR, you're probably best to imbue it as low as possible. If not, then imbue whenever you like, accepting that at higher levels some bad affixes come into play, much like armour. There is the obvious point that a 3Dpavise/tower or Sigon's Guard is more likely than not better, however it's possible to generate better shields than those, if you're not looking for just high resists and/or blocking. You could produce a shield, at level 24, a shield with 29% lit resist, 29% fire resist, Magic Damage reduced by 2, +59 to life, fastest hit recovery and deflecting. However, there does come the endless point you could just gamble a good shield this way, too. So it's up to you. It's odd that rare shields aren't commonly heard of on BNet, when this is indeed one of the strongest slots. Of course, Smackadins would want a rare barbed shield, which doesn't show up commonly at all. Personally, for my hammer/smack paladin (odd imagery there) I intend to produce an imbue mule and imbue at level8, and hope for deflecting, might and fortified.

Imbuing Gloves:
Imbuing gloves has only two great modifiers: the Perfection series and Slightly increased attack speed. Other abilities generally aren't that great; in fact, Slightly increased attack speed isn't that great either. You might want +magic find, but those occur often enough that it's probably better to gamble or just wait for a drop, or wear Chance Guards. So, personally, I'd recommend you don't imbue Gloves, but if you are dead set on using up your imbue like this, I would recommend either 29 (at which point you can get Holy, Perfection, All the +magic find abilities and of the Tiger) or 49 (where you can get the Garnet, Cobalt, Coral and Viridian resistance affixes) depending on what you want. Also, Light/battle gauntlets with the Mammoth Suffix look absolutely wicked.

Imbuing Belts:
Imbuing Belts is a fairly safe bet, if you're just looking for a generally 'good' belt. There are a lot of affixes on belts and they could have just about anything. Also, belts receive the +DR% series at the same time as armour, so if you're going to imbue, one of the Exceptional belts is a good idea since you'll at least get some useful DR that way. Keep in mind that exceptional belts all have 4 slots, so even a Demonhide Sash can be fine. The 'best' belt modifiers for most classes is probably of the Titan (24) Colossus (24) or Stability (3). Also, you can get the 3rd level resistances (21-30%) on Belts as of 12, except Viridian (which shows up at 49 ?). So clevel 24 is probably a good time to imbue a belt, if you want. Although it's possible to gamble just as good a belt, you can't gamble War belts and an extra 40-80 DR over a Plated Belt certainly won't hurt.

Imbuing Boots:
Boots, are a rather risky item to imbue. It is both a very powerful slot and a very weak one - it's possible to get boots with 3 of the top level resists (lightning, fire and cold), +19 to STR and Fastest Run. That would certainly be a nice boot, now wouldn't it? However, boots have the very bad problem that if they don't have faster or fastest run, you are probably not going to want to wear them. Fastest Run opens up at level 16; in fact, I used boots on my characters as an imbue mule at this level (he had no other use) and got fastest run, which was pretty nice. After that some other nice affixes are Ruby, Sapphire and Amber (all at clvl19) , Dragon's (45), Giant (46) and Titan (24). Stability opens up at (3) Fortune at (10) Tireless at (8) and Tiger (14).

Conclusion:
I've looked over the affixes and give you a very general set of ideas based on the affixes possible on the item. This doesn't say which is the best item to imbue - there really isn't, and a lot of it is luck. If you want to imbue an item you can't get your hands on, and you're on BNet, it's usually pretty easy to buy exceptional items like Gothic bows or Executioner Swords. Some of the other stuff may be hard to get your hands on at the lower levels, but that's all a matter of the 'flow of the game'. There are a few myths about imbuing - One is that your normal, nightmare and hell imbues are somehow different. This isn't true; I've done scores and scores of testing and never seen any real difference. Another is that Superior items imbue differently, which I don't have solid evidence on but I've never noticed a difference. The last is that imbues get better as you reach higher levels. If you read what I wrote, you might realize this is half true. But a level 70 weapon imbue could have regeneration it; I'm sure we'd all be pleased to get that ... :(

My report is based on the data provided on the CS and the not so well known +6 to clvl modifier on imbues (which a few people have pointed out, testing proves this is true). I'm not infallible, take every think I read with a bit of caution - remember, Blizz could change any thing whenever and we'd never know ! :)