5th. The N64 and PS1 were so different yet both so good that you really needed both to experience the best as each had an abundance of exclusives. It was also the most radical leap in gaming in my opinion.

I'll take the 5th generation. 3D was a huge thing, the generation helped a lot in popularizing gaming, and things were still fresh and devs tried new and different things. The 6th generation was largely the same, but much of the innovation was already gone, and a lot of games released back then were just polished evolution of the games we saw during the 5th generation. Don't get me wrong, I love the 6th generation, but in a way, it felt a bit stale and more commercial compared to the 5th generation.

Of course getting into gaming during the 5th generation might affect my feelings, but I'm doing my best to not let them affect my answer!

On a side note, the question is a bit ambiguous. Depending on what 'relive' exactly means in this case, my answer could change.

6th Gen cause it had the greatest console of all time (PS2), and I have access to the PS1's library through BC.  Then there's the GBA, which was great as well.  I would say 5th, but the PS1 was the only good console in terms of variety and 3rd party and the GB/GBC's libraries could be played on the GBA, which had a great library itself.