This is a guide for warriors for gearing up, though paladins may want most of this. Death knights will need to tweak it a bit, especially the the weapons and shields, and druids obviously can't even equip most of this. Feel free to add lists for other classes, or other warrior gear to this list. It is a wiki after all! ;-)

The first list shows gear for getting your stats up quickly to a respectable level. The second and third lists show some upgrades you should watch out for. There's no heavy theorycrafting involved; there may be other gear you prefer.

Note: Tanks need 540 Defense (140 + base) to be uncrittable. That is 689 defense rating (140 x 4.92). Enchants can account for up to 58 rating. You can use resilience to close the gap, but warriors and paladins will want to switch it out for defense as soon as possible. (I'm not 100% sure about druids and death knights since I am not as familiar with their mechanics.)

Note Druids: Druids do not need to worry about defense as with talents they are un-critable. In fact there are no leather items with defense on them after Burning crusade thus their mechanics for tanking are different from the plate classes in regards to stat prioritization.

Tier 1

Quests, Reputation (up to Revered)

The Tempered Saronite gear is a good set to start with too.

Tier 2

Normal Dungeon Drops, Exalted Reputations, Rare BoE and Crafted Items

Tier 3

Heroic Dungeon Drops, Epic Crafted Gear

* Not as good as the Heroes' gloves, but something to tide you over until you get enough badges.
~ Close to the Azjol-Nerub belt, and a good buy that doesn't rely on random drops being co-operative.
^ Not quite as good as the Emblem shield, but this will tide you over while you buy the other stuff first. Also a great entry item for getting your tank stats up.