Assuming we put the limit at minimum 3 games, and Dark Souls 2 doesn't suck, the Souls series blows the competition out of the water.
- The series has grown huge solely on the back of its excellent and unique design, not due to a massive budget or marketing campaign. Demon's Souls was released as a PS3 exclusive without a planned NA/EU release, but caused such a massive buzz online and so many people imported the game that it was re-released in NA/EU due to popular demand, leading to a large cross-platform release of the sequel
- Singlehandedly bringing back uncompromising gameplay to the AAA scene
- Revolutionary online functionality, which still allows you to play offline without crippling the game
- Extreme replayability and near infinite difficulty scaling (through self-imposed challenges, made possible by an extremely high skill ceiling)
- The first two games are both very good, the third is getting almost universal praise in pre-release press (We all know how reliable that is, though)
Uncharted and Mass Effect are great, but both are mostly well executed evolutions of existing AAA games. Assassin's Creed was unique and innovative, but let's be honest - the first game was pretty boring past the first couple of hours, and there have been a few duds since. I'd say only AC2 and AC4 qualify as "great games".
Assuming DS2 is good, I'd rank them:
Souls > Mass Effect > Uncharted > AC
Portal would be a contender for the top spot if there were 3 games.