GTA for me. I was thinking Pokemon but then I thought of all the new monsters. C'mon Nintendo...a hundred and fifty new pokemon every year?? I'm starting to think that those monsters are inter-breeding with all those new species of pokemon coming out.
Pokemon was probably my first franchise I fell in love with. I remember having yellow version and because I wanted to start a new game, but didn't want to delete my save data (they really should make more than one save slot) I stole my friends copy. To this day I will still play Pokemon.
Two other franchises that are right up there with Pokemon are MG/MGS (anyone excited for Metal Gear Rising: Revengence?) and Resident Evil.
I'm a Sims guy myself, and I also love the Roller Coaster Tycoon series, even though both franchises seem to have been going downhill ever since they both released their third installments.
for me, right now probably CoD (don't hate/judge me because of what MW3 turned out to be) i'm getting into battlefield though, but one of the big reasons why this isn't number one is because i've only played BF3. however the game that i've been the most obsessed about would probably be the star wars battlefront games, but i've outgrown those for the most part
Pokemon. When i was a kid, I just loved those games. LOL sometimes i didn't even sleep just to play those games. My 2nd and 3rd favorites are Mario and Digimon.
I would have to say Dynasty Warriors. Even though I haven't played any past the sixth, I had all of them for the PS2 and played them regularly. I especially liked the Empires ones, where you have a giant map and try to take it over. But it wasn't as simple as that, there were many other features involved.