This might be a copout, but I like both equally, for different reasons of course.
Retro: many retro games have a special charm to them that is absent in most modern games. In addition to that they are simple yet fun most of the time; there isn't tutorials or giant, overly complex gameplay mechanics, just you and the game. And of course, Nostalgia makes retro games just that much more enjoyable as they remind me of being a kid again.
Modern: modern games are mostly a huge leap in evolution over their retor counterparts and this can be for better or worse. I also am blown away by huge games like GTA or Skyrim. I definitely find myself more immersed and dedicated to modern games than retro games.