The undoubtedly EASIEST game to make would probably be a text adventure. No physics, no real-time calculation, just if they type this then do that. If they have that in their inventory, do this.
The hardest part would be designing all the puzzles, and maybe making an easy to use command system; but there's really no need for a full blown commercial-quality parser that can understand complicated sentences. Just have several basic commands like:
OPEN
GO
LOOK
etc. Then after that do PONG, then something cool like an FPS or whatever you want.
Juvenile Industries
Current Project: KillZone(FPS)
Soon to come:An rts, and a snowball fight game