That's a tough one. I can think of examples for 1st, 2nd, and 3rd:
Peaked at 1st game — F.E.A.R., BioShock, Dead Space, Metroid Prime, Tomb Raider (Core Design), Batman: Arkham, Infamous, Halo, Jak and Daxter, Xenoblade, Ninja Gaiden (Black/Sigma)
Peaked at 2nd game — Uncharted, Fable, Bayonetta, Tomb Raider (Survivor trilogy), Resident Evil, TimeSplitters, Zero Escape, Doom (modern trilogy)
Peaked at 3rd game — Darkstalkers, Street Fighter Alpha, God of War, Gears of War, Metal Gear Solid, Ratchet & Clank, Super Mario Bros., Darius
Going in, I guessed the third game would be the winner, but I'm not sure now. Also, I'm certainly overlooking some trilogies (and unsure if the listed ones all count). I suppose if I had to choose, I would pick the first game.
