I had to stop doing pet battles or I never would have levelled my character. I got a few pets into the level 7 range and wanted to continue fighting the trainers, but i would have needed to level my pets more before each trainer, and that would have just eaten up all my time. The most amazing thing, to me, is that I absolutely hate Pokemon. The game can't keep my attention for more than 10 or 15 minutes.
So far my favorite pet is my mini-corehound. I also have a pet, can't remember which one, that does a move that causes his next attack to do double damage. One of his other abilities is an extremely hard-hitting move. Pairing those 2 up is almost always a 2-round kill. Haven't looked at my battle pet pane recently, so can't remember which one it is.
I have a feeling that having varied teams is eventually going to be the way to go, just so you have options depending on what you are fighting. Once I am level 90 I will go back and do more pet battles. I want to trap every pet possible.