Series that "start" with a sequel?

Geometry Wars comes to mind. The first game was inside Project Gotham Racing.

Would Halo 2 be a fair entry? I know the first one was massive with gamers, but I feel like with 2 having online play, everybody was talking about it. Friends who only really played games casually got into it, and it was really a landmark for console gaming in some ways. Maybe I'm off base though....

I'd say Gears 1 and Uncharted 1. They are both mediocre as hell and their sequels improve in just about every way possible.