What are some good tabletop R.P.G.s?

I have a gaming group with my friends and their siblings, with ages ranging from 12-18. It works sometimes but lately we haven't actually got anywhere during gaming sessions. As the GM Ive noticed that everyone wants something different from the game, and no one is really into playing Dungeons and Dragons as much as goofing off. I'm looking for a System that's simple enough to get everyone together, and structured enough to make sure we don't fall apart. I'd be delighted if someone could find a good system, official or home brew, any setting, and any price, that will help keep the group together. Thanks for reading.