I must be one of the crazy people that does fine with the default UI when on PTRs or something.
While I *want* to use addons, I don't *need* them. I'm sure that there are a lot of things people listed above that they just "want" and don't really "need".
But here are the things that I always get first for my addons folder:
some bar mod (I have preferences, but as long as I have something)
some UF mod (^^^)
Clique
BugGrabber/BugSack
RatingBuster
TomTom
OmniCC