I can't stand playing with the default UI frames, it just won't "work" for me, so:
oUF_Phanx. I love how the buff system operates, it means I have access to vital information without unnecessary cruft.
Also,
AdiBags because I am terrible at housekeeping. I get out of combat bag tunnel vision without an addon like that installed. I used to use Baggins and ArkInventory also.
And offcourse, Bartender / Dominos.