Sonic.

Too easy - the franchise that made me a gamer - Final Fantasy.

This is really tough choice between: Heroes of Might & Magic, Pokemon, Ratchet & Clank, Metal Gear, Uncharted, Total War, Dawn of War, BFME, InFamous, WipEout, Rayman, Batman: Arkham franchise, God of War, Tekken, and so on...

My answer is that I don't have a favorite, but if it doesn't count I'll say Heroes of Might & Magic for the nostalgia.

Edit: Picture added

The Legend of Zelda

Metal Gear Solid

Fallout

How about an update:

1. The Legend of Zelda: 21 users
2. Final Fantasy: 12 users
3. Metal Gear Solid: 7 users
4. Pokemon: 6 users
5. Super Mario Bros: 6 users
6. Metroid: 5 users
7. Kingdom Hearts: 4 users
8. God of War: 3 users
9. Grand Theft Auto: 3 users
10. Halo: 3 users
11. Mass Effect: 3 users
    
    

Well that's the ranking so far...But take note that Pokemon/SMB are tied for 4th and are arranged alphabetically.  Same goes for the final four entries.

Any surprises here for anyone?  

God of War. Hard question, but that's it.

The Legend of Zelda

I'm surprised that nobody answered Age of Empires or Warcraft, GTA should be higher on the list, and Mario and Zelda should switch places.