Yu Yu Hakusho
Although, then again, if I was only going to be able to watch 1 single anime for the rest of my life, it would probably be better if I pikced a long one, instead of my favorite one, just as lon as its entertaining, because I would want something that could last me an extremely long time. If that were the case, then I would probably go with either One Piece or Case Closed (both of which are still continuing, if I'm not mistaken).