That's really too tough to answer. You limited it to "Solid" games, though, and that helps. I just played Metal Gear one with my MGS3 LE disc, and I was blown away with how much was included in that. Kojima has reused a lot of elements from those games. The cardboard box was in it! So many weapons, too, I think most of them that were in the MGS1 game. It's crazy. I'm going to play MG2 soon as well.
I'll have to go with MGS3. The Boss fights were more unique (turns out a lot of the same boss fights were in MG1 that were in MGS1...although there was a fire shooter in MG1 too, heh), and the graphics are of course improved. Maybe Twin Snakes will change my mind (gonna pick that up when I get my Wii).
I also really enjoyed MGS2, despite Raiden. I didn't dislike him as a lot of people do, I just missed Snake. It's weird, because after MGS 1 you never get to play Solid Snake the same. In 2 you are Raiden, in 3 you are Naked Snake, in portable ops you are Big Boss, and in MGS4 you are old snake, apparently. MGS2 was still awesome, imo, though.
I haven't finished Portable Ops yet, either, so maybe I'll change my mind. If only my girlfriend would give me back my PSP...*sigh*
