Unitframes - Use PitBull. Most customizable frame, whitout being as confusing as some of the more lua based addons.
Minimap - Chinchilla. Great customization, and nice looks.
Action Bars - Bartender's the only one you need to know. There are Dominos, but i'd always choose BT4. And you should add ButtonFacade, for nice looks.
Castbar - Quartz is the only thing worth getting.
Buffs - Here it's up to you what kind of buffs you like. If you like the way Blizz set up your buff as squares on rows, get Buffalo, same style just customizable. But, if you want your buffs to appear like bars, then get ElkanosBuffBars.
An SS of my UI, using PitBull, BF, Chinchilla, BT4, Quartz, EBB, and various others.