I use:
ElvUI
ElvUI Kkthnx(includes ElvUI Location Plus, ElvUI Still Enhanced, cargbags, and a bunch of other customizations to make it a bit more like RealUI)
ElvUI AddOnSkins(makes a bunch of popular addons match the rest of ElvUI)
DBM
GTFO(still necessary even with DBM imo)
WeakAuras2(another must-have for every class, as long as you can find some good strings for it or know how to make your own)
Skada(much better than Recount now)
Clique(for when I heal, fuck making a dozen @mouseover macros)
Pawn(with stat weightings I generate myself from SimC, but you can probably find good ones on IV after the xpac is out a while)
_NPCScan(gimped in 6.0 but it will still find and announce vignette mobs in MoP and WoD)
TSM(I'm on a server that takes 3-4 hours to do a full AH scan, so I rely on TSM AuctionDB and WoWuction getting populated by the desktop app for my AH data)
Auctionator(even if you use TSM it's still nice to have a plain old AH mod too. Auctioneer is too bloated, but Auctionator is perfect for the job.)
BadBoy,
Badboy_Guilded, and
BadBoy_CCleaner(anti-spam shit)
BagSync(lightweight addon for keeping track of stuff across alts. Altoholic is a good choice too if you don't care about the lightweight part.)
BugGrabberand
BugSack(need something to manage UI errors, especially with how often the Blizzard UI gets tainted these days)
Foglight(map addon, gets rid of fog and shows adjacent zone terrain too)
Everything except for ElvUI Kkthnx is available from the Curse client. Grab Kkthnx from
WoWInterface.
P.S. Omen hasn't been needed in ages.