Historically speaking, I think the most interesting system is the Apple II. There was just so many groundbreaking games that originated on that platform, especially role-playing, adventure, and strategy games (my favorite genres) as well as some action games. Plus, it was around so long that you could collect models, accessories, upgrades, etc., for years and years and years. On top of all that, you could go for uber-collectibles like Akalabeth or Mystery House.

As far as consoles go, I don't see much choice there, unless for whatever reason you really hate Nintendo. Either the NES or SNES would give you incredible variety. I guess if I had to choose either/or, I'd go with the original NES.