Up until a few years ago, I'd have said single player computer games, but I joined a great group of experienced pen-and-paper players who prefer 'alternative' RPG's (more story, less math). Currently playing two games, one set in a post-apocalyptic Toronto with a mysterious, possibly sentient, mutagenic mist, and the other searching for Nazi/Alien 'supertech' in a paranoid post-WWII cold war Berlin.
I've literally played ALL of these. And all of them have years poured into each kind. But I think overall offline single players have had the most love from me in my life. Why? Because they where there first in the consolue/PC world back when I was a kid. MMO's wheren't big til the past decade and even those where meh until Warcraft. When it was released THEN MMOs became really popular and got people playing IMO. Even when I was playing MMOs I would still occasionally go back and play the original games, which where single offline RPGs, or just some other game like Neverwinter or Alice or Diablo. Although honestly I have been back at doing text/forum RPing. lol