ui totally depends on ur preferences. u can revamped the whole interface or jus apply the ui that u need 4 ur class. u may chk out http://www.curse-gaming.com/en/ for the ui mods.

some of the ones im using are:

Titan Panel - A info panel that sits on top n bottom of the screen (highly recommended)
Titan Aggro - who's being aggro in ur pt or raid.
Arc HUD - a useful hud that displays health/mana/cp/rage on the middle of the screen.
Natur EnemyCastBar - lets u know what magic the enemy is casting.
Spell Alert - same as above but in text form.
MobInfo-2 - keeps a database of the mobs u've fought, n what they drop n the % of the drop.
Auctioneer - scans the value of loots in auction.
CT Raid Assist - must have for raids.
Guild Event Manager (GEM) - this depends on ur guild.

if u're into total interface mod, the i could recommend u to try Discord Action Bars, CT Mod, Perl Classic Unit Frames.

hope it helps.