I don't know if you are looking for a complete UI or just some addons, but in my opinion it helps to start of with a good ui.
As a base, I now lately started using TukUI.
To add on top of it I use:
- !BugGrabber : Makes sure I don't get annoying popups about addons causing errors when they do, instead logs it to the chat pane
- Afflicted : I use this in PvP to track the cooldowns of oponents so that I know when to be ready to prevent a problem
- BaudSpamFilter : Prevents 90% of gold spam in general/trade chat and some whisper, but I found I need to block whispers from level 1s to block those
- Broker2Fubar : Mainly to just get WIM on to Fubar instead of on my minimap
- ClassTimer : I use this on all classes to track debuffs on my target as well as buffs on friendly targets ect.
- Chatmod : Make chatting a bit better
- DBM : Everyone needs a BossMod, although I see there is a new great alternative called DXE encounters @
http://www.wowace.com/addons/deus-vox-encounters/
- ErrorMonster : Remove the RED text in the middle of my screen, I mean I know I spam my abilities when I do not have enough rage/energy
- Gladius : <ust have for Arena
- Mutilate : Sometimes I am too lazy to remember to keep up buffs/debuffs this makes it easy for me while raiding so I can watch TV
- OmniCC : Puts a cooldown count on your buttons, but give it in seconds instead of minutes
- Postal : Great addon to simply opening 100000 mails from the AH
- PowerAuras : I could use this more but mostly it shows a few important abilities only
- Sapper : Again for PvP and arena to say when my sap/blind gets broken by trinkets or someone doing damage to the CC'd person
- Skada : I use this in place of Omen/Recount ect to show my tables for Aggro and DPS, although Tukui already has a threat meter bar
- TipTac : My favourite tooltip frame replacer
- WIM : Makes chatting easier in WoW, alot like having MSN inside WoW
And here is a screenshot doing going into a Arena:
http://www.restricted.co.za/upload/WoWScrnShot_021310_114501.jpg