Phantasy Star Mass Effect Resident Evil Devil May Cry System Shock Transformers Star Wars Fallout Castlevania Mega Man
I'm kind of cheating with a few of those responses, since I like almost anything released in their respective franchises and not specific series as a subset (for example, the Jedi Knight series for Star Wars.) In no particular order, too, other than Phantasy Star topping the list.
1. Metal Gear (between the main series, spin-offs and Online 1 and 2 the best there is) 2. Grand Theft Auto (I remember playing the demo for GTA1 the series just gets better) 3. Castlevania (2-d only though) 4. Demon's/Dark Souls (2 games in and it's already made the top 5) 5. Versus Series (XvSF, MSHvsSF, MVC1,2,3,etc) 6. Ace Combat (though Assault Horizons was my first disappointment) 7. Phantasy Star (Universe is the only one I haven't liked so far) 8. Soul Calibur (played and finished them all) 9. Street Fighter (same as above) 10. Need For Speed/Burnout (now that they are one in the same it's great for fans of both)
1. Mortal Kombat (old and new) 2. Street Fighter (old and new) 3. Ninja Gaiden (current gen) 4. Sonic the Hedgehog 5. Halo 6. God of War 7. Devil May Cry 8. Max Payne 9. Grand Theft Auto 10. Batman Arkham