When I think of a series that peaked with its first game, I think immediately of Jak and Daxter. The first game was an easy to pick up and play platformer with bright and inviting levels and is still one of my favorite games ever. The later games took a dark turn. Perhaps that's why we don't hear from Jak anymore.

Most successful series go the other route, where game 1 often falls far short of the best game in the series. In an ideal series, game 1 would be the worst as games improved on one another each time. Super Mario Bros. is a fine game, but it pales in comparison to SMB 3. It's amazing how much they were able to cram into SMB 3 and how primitive 1 looks in comparison. SMB 3 just feels so much more fleshed out, so much more substantial. Super Mario would go on to maintain that high standard through future games. While Super Mario 64 was the first 3D game in the series, in my opinion it was the best of all. But then again, being born in 1992 may have something to do with that opinion.

Sonic the Hedgehog is another prime example. Rerez put it perfectly: Sonic is slow in Sonic 1 compared to later Sonic games. Sonic 2, on the other hand, is a masterpiece. Marble Zone just didn't work out (despite the kick ass music) and Sonic 2 had much more of an emphasis on speed. I feel like Sonic didn't do the 3D transition very well, the 2D games are the best, but at least in its early years Sonic was improving. I personally enjoy playing Sonic 2 more than any other Sonic, but they did continue to introduce good ideas into the series with 3 and Knuckles. The save system from 3 is something that they should have had all along, I like the graphical style in 3 and Knuckles more, and I always enjoyed playing with Knuckles in the Sonic 2 and 3 levels. It wasn't until 3D Blast that the main Sonic series went downhill.