On the heels of having had, shall we say, "lively" discussions with Master Looters over whether two belts were "best for healers. healers get first priority" in Ulduar in the past two weeks (once in 25 the other in 10), I realized the best defense is an offense. These discussions go a lot more smoothly when you can point to unbiased third parties and say "um, no."
I'm not a math geek so I am not going to argue BiS (best in Slot) with the maths here. Rather, I am going to discuss your gearing up opportunities by boss, to help you prep your wish list. If you want to see BiS across all available 10-man and 25-man raids, including Ulduar hard mode boss drops, shadowpriest.com has an excellent list. With more math than you can shake a stick at.
Words of Caution: Much of the Ulduar loot lacks hit. So don't be quick to shard or sell your existing gear when you receive an upgrade. You may have to swap gear around for raids to ensure you stay hit capped.
The list, after the break.