i have to go with starcraft2 2 reasons...

1. cause they are taking a good thing and improving it instead of changing it... if it aint broke dont fix it and starcraft is still the most played rts to date... and its 10 years old!! even with dow out it still wins.

2. Its blizzard for petes sake... they have set all kinds of dang records, and obviously know what they are doing.

oh yeah and it doesnt really matter cause no one said you could only own and play one lol... i will own them ALL, and PLAY them all mwa ha ha ha def looking forward to some tyranids too baby... and some vamped space marines!! assault marines ftw