For unit/raid frames I use Pitbull. I use Chinchilla for my minimap for the handy little 'position' mover that lets you move the quest tracker, durability, capture bar, world state, quest timer and achievement tracker anchors. For action bars I use Bartender4 with Button Facade. SunnArt is great for on screen art (even has art packages) and viewport is nice. For buffs I use the standard Pitbull aura system with custom border art.
This is my UI:

Out of combat.

Everything showing with party frames.

Everything showing with raid frames.
I like minimalistic with color, which is where the art comes in. I also like things to
look like they belong in Warcraft, which is why I don't like many minimalistic UIs because they use pixel fonts and things look futuristic or spacey. My UI is a neverending endeavor, and I'm still looking for a good damage/healer meter.
If anyone is curious, these are all the addons I'm currently packing: Align, Bartender4, BaudBag, Button Facade, Button Facade: Entropy (Copper is the skin used in my UI), Cartographer, Chatter, Chinchilla, CowTip, DoubleWide, EnhancedColourPicker, GFW_Levelator, MiniChat, naiStats, oGlow, Omen, Pitbull, Quartz, QuestHelper, SharedMedia-Blizzard and SunnArt.