I played through Halo 1 and 2 co-op just prior to playing through Halo 3 and I think they've held up well. In fact, I liked Halo 2 better the second time. I've played all the way through Halo 1 I think 3 times now and I've enjoyed it everytime. It gets boring around the middle (the Library straight up sucked), but there's enough great parts throughout the game that make it worth it. The second I feel is worse, although it has its moments. The third was even a further step down, and had few really good moments.
The key is co-op, it makes a huge difference. I'm not sure how well they hold up single player. Oddly enough, I felt the first one was actually the funnest, despite co-op being very tacked on. Oh, and also I think it had the best difficulty. Legendary co-op made the first game more fun, while it was too difficult in Halo 2, and Halo 3 was too easy.
Story-wise, I liked the first, as the second was a pretty big letdown, and the third's story just never got good at all in my opinion. There were some fun moments, but I think Bungie really screwed up the story and I absolutely hated the Gravemind. It had so much more potential and really should have dropped the Flood after the first game. I think in the first game, the story mostly got out of the way and let the gameplay take over.