I like alot of Pokemon games. Some more than others, but I really liked the Yellow version. Personally, I think the older versions were harder to beat than the newer ones, even though they're all pretty easy. Emerald was fun, too, especially with all the Pokemon competitions, so it was a nice gaming experience. LeafGreen and FireRed...well, they were OK-ish. But they were sort of just revised versions of older games, so it wasn't like a totally new gaming experience.
For the newer Pokemon games (Diamond, Pearl, Mystery Dungeon, etc.), I like Diamond and Pearl (even though I've only played one of them, but based on what I've seen, they aren't so different).