We've used Avent, Tommee Tippee, Similac SimplySmart, and Dr. Brown's. The Avent and the Similac bottles were samples we received; I HATED the SimplySmart bottle, I've only used it once and not again since then. The Dr. Brown's bottles were okay, but there's a lot of "assembly required" (there's like, 4 different parts to it) and the flow seemed too fast for my LO. We alternate between the Tommee Tippee and the Avent bottles, and I like them both.
The TT bottle has never leaked for us, the flow is appropriate and LO likes the nipple. DH doesn't like the shape of the TT bottles, he says it makes it harder to hold (they work fine for me), and the nipple is a lot more "collapsable" than other nipples. Also, if you use a bottle warmer, TT bottles do not fit in most warmers, and their bottle warmer has gotten less than stellar reviews.
The Avent bottle will leak if you do not have the nipple part screwed on properly (I often have to unscrew and re-screw the cap on), and if you lose the internal ring (you'll know what I'm talking about if you get one) the bottles will be useless because they'll leak all over the place. The shape is more DH-friendly, and the nipple does not collapse like the TT bottles (not saying this is a good or bad thing, just different).