I use both Trinity and Bongos3 (Trinity for when I want a more complex UI and Bongos for when I just want to start and go), so those are both high on the list, though I would not really consider them my "Favorite"
I think my Favorite AddOn would be MTLove (Now MTLove 2). It's probably the oldest AddOn I've had, and I only need to update it maybe once a year. It simply shows in a tiny little box who the mouse-over target is targeting. The reason I love it so much is that the first character I leveled to 60 (And then 70), was my Protection Warrior. This AddOn made tanking SO much easier, just swipe my mouse across the targets, and if it doesn't say "Valdenar" on one of them, I find that one and lay the smack down.