Losecontrol - cc timers for self, target, focus, group
GladiatorlosSA - pvp triggers/CDs sound announcer, extremely good, you can configure what it announces and add custom stuff
Capping - bg timers etc, dbm pvp mod both are ok
BGEnemies - enemy frame bars
Gladius for arena
Healers have to die (hhtd) - if you want healers marked
OmniCC
Quartz - good cast bars
TellMeWhen and Needtoknow, but not sure if they're updated, havent used since my arena days
Semi related also,
Dejacharacter stats
Bartender 4
Leatrix plus
Moveanything
Mik's scrolling battle text, you configure and show what you want including custom triggers
Threat plates nameplate, extremly flexible, configurable
Z-perl unit frames, personal favorite but you can use what you like
Don't use premade ui, only amateurs fall for that.
Make one yourself and research and do it well and go from bottom up, same goes for practice, you'll be a better player.