I abhore those addon collections (ie CTMod, Cosmos, etc...) Half of those things you won't use, at least.
CTRaid - Essential if you get into the high end raiding dungeons.
Auctioneer - Good for knowing the current going rates for auction house items.
Titanbar - Most of the important info easily displayed across the top and/or bottom of your screen. Gold, bag slots, durability, location, honor, XP, etc..
Gatherer - Keep track of where you have found mines/herbs/chests and marks them on your world map so you can find those rare spots easily.
A couple not so essential mods -
Flexbar - If you don't like the UI layout and/or want some additional functionality, look into this. Having some coding background really helps when setting up events and whatnot, but you can program the 120 buttons to do a lot of cool stuff.
MoveAnything - Change up the entire UI! Or hide parts of it that you don't need.
Decursive - Must have for priests, but the other classes with "cure" spells can benefit from this tool. One button press to cleanse members of your group.
Necrosis - Warlock tool, although the latest versions of it are not as good (IMO) as the older ones. Tracks soul shards, spell timers, has the warlock casting bar, among other things.