Gear is irrelevant until level 80.
A list of best pieces for each set of levels is pointless because of how quick levels pass by.
What you generally want to do, is look at the piece you currently have and a possible upgrade.
Compare their stats (ignore anything they both have) and see what you gain and/or lose from switching a piece.
Then figure out how much each side is worth to you and if it's worth it.
Lets take some imaginary items, lets say trousers, imaginary trousers are cool.
Piece one is called Ghostly Pants, piece two is called Leggings of the Lich.
Ghostly pants have 15 spellpower, 7 intellect, 5 spirit and 15 stamina.
Leggings of the Lich have 10 spellpower, 15 intellect, 10 spirit and 7 stamina.
Now to compare them:
GP --- LotL
15 SP. 10
07 int 15
05 spi 10
15 sta 07
Both pieces of equipment have at least 10 spellpower, 7 intellect, 5 spirit and 7 stamina so we can ignore that much.
That makes it 5 Spellpower and 8 stamina VS 8 intellect and 5 spirit.
We can see that both pieces are quite equal in itemlevel and statvalue but focusing on different things.
The Leggings of the Lich will give you more mana and mana regen, which is great if you find yourself running out of mana but not of that much value when you stopped thinking of mana a while ago.
The Ghostly Pants have a lot of stamina that "wastes" itemization, if you're only doing instances that stamina isn't going to do you much good, but they also have 5 more spellpower, which helps your damage/healing throughput.
So we have a few options now and you'll have to think about it.
A: You are fine on mana and taking the Ghostly Pants.
B: You want more mana/mana regen and take the Leggings of the Lich.
C: You are fine on mana, but you don't want or need the stamina on GP so you switch out some other piece(s) of gear for more spellpower and less mana(regen) and still take the Leggings of the Lich.
D: You could use more mana regen, but you are feeling quite squishy so you go for the Ghostly Pants and use other pieces to get/keep mana(regen)
Edit: Obviously, the pieces mentioned above are really close in ilevel and strength.
While leveling, most gear decisions are easier as one piece will have much more stats than the other, making the choice obvious.
Doing this small comparison however is a good way to figure out what piece you want, and doing this before an instance is a good way to ensure that you know what you want to need on.
Once you get some practice with comparing you'll find that you do it on a glance and can figure it out quickly while looting (and if you can't, the difference is often so small that it's not that important)
Edited, Feb 3rd 2010 12:57pm by Aethien