The best game - Final Fantasy 7. Game is just amazing from start to finish. Obviously if one plays it today without having ever played it before, it won't have the same impact. With that said, features like the materia system and the story don't fade. These 2 are still the best in any JRPG and, generally speaking, any other RPG as well. I mean, these characters have been in multiple games outside of FF7 the original since then, including a movie, Kingdom Hearts series (Sephiroth is the hardest boss in all KH games, and normally a "secret" boss). It's a huge game, and I'll love it when they remake it someday.
The worst game - I'd probably say Halo 3. I like shooting games (well, these days not too much) but I liked them when I first played Halo 3, and I was extremely disappointed. Graphics weren't that good, story was okay but not that interesting. The main thing that killed this game for me, is I just got done playing Gears of War (first game) like a month before this came out. I went from playing a very fresh, very fun game where cover was never done before like that in a game before, to an average shooter in Halo 3 that played like the first halo 7 years before it. Left an extremely bad taste in my mouth. I haven't really liked Halo since then, because I've played far too many shooters in my time and it takes a unique shooter with unique game mechanics to really get me into it - that or very good co-op, like Gears 3.