Like Jared, I'm about 50/50 lately. For about 14 years all I would use was either a 3" flat for interior, or a 4" flat for exterior, but recently I gave the angled brush another shot and I have to say I'm becoming a big fan. I've found the 2.5 angled to be a great brush for windows,door casings (especially the sides), or cutting in walls where the owner doesn't want the trim painted, just a fresh coat on the walls. Each have their own set of places that yield the best results.
So to answer this thread, I would have to say 3" flat, with a 2.5 angle gaining ground.
BTW, I like the Wooster or Purdy (firm and soft, depending) for the 3" flat, and the Corona Vegas 2.5 or 3" angled.