Of the top of my head, my top series are:
Mario Series: Although I haven't played any Mario games since Galaxy, Mario games have always been the best and only choice for versus gaming. There's nothing like a bunch of cousins cursing each other and going all out on Mario Kart or standing up from excitement with the mini-games in Mario Party. Single player Mario games are always fun too, Super Mario RPG is my all time favorite.
PKMN: I fell way behind with Pkmn, barely finishing Diamond. Haven't played the later gens. But no other games have more of my playing time than pokemon. Even if its not my favorite rpg, it's probably the most addictive.
Final Fantasy: I recently decided to get back into FF. Oh how I've missed it. My Xbox broke down, so I was never able to get the FFXIII that I wanted so much. About to start FFVI Advance. I'm a big fan of the game stories, and I'm all about completing the game at 100%, with every item obtained, every dungeon passed, even the completing the bestiary. Indeed my favorite rpg.
Halo: As far as favorite game or series go, Halo is it. A little mainstream, sure. But I think I know more than I should about Halo. I'm not talking about knowing how to play, no, I have only average skill. I'm talking about being a big fan of the story behind Halo. Either way, the game is awesome. Multiplayer is ok, but there's nothing better for me than playing campaign on the big screen when you have the house to yourself. Must be a shooting game thing.
Ok so this wasn't off the top of my head, I got carried away.....my bad....