If we're talking about just the characters, then probably Finn and Jake. They take down things way more threatening than Mordecai and Rigby on a daily basis just for fun.
About the shows, I'd say it's a tie for me. I like and dislike things about both shows. I really like Adventure Time, but the randomness behind the humor is starting to grate me a little. Regular Show, on the other hand, is pretty okay for the most part, but there are some characters I dislike a lot, which makes the show a bit difficult to enjoy.
Then again, I haven't seen that much of either show, so I really don't have that strong of an opinion yet.