Apparently these two from Viva Pinata.
But no, I'm not
completely sure, but I think it's the achievement that sold me on the very
idea of the achievements system in the first place; waaay back at the start of the 360, I always liked the idea of an achievement based around fundamental changes to your playstyle, and there was one iconic achievement that cropped up in the very earliest wave of titles that stood out. I didn't actually get my 360 until some time later, but that achievement always stuck with me and I was always going to pursue it once I could:
(I should mention that I also have some Bioshock achievements tied to that first day of play, and PGR3 Rank 10, but I *think* it's Geometry Wars I made the initial beeline to)