The series has constantly improved upon itself. Every generation has made the gameplay experience better, and that's kinda why I haven't stopped playing yet. So, I'd say Gen V is currently Pokemon at its best.
R/S/E and FR/LG were the best times for me. I had a bunch of people to play with back then, the games were new and I knew nothing about R/S/E going in, so it was all about exploring and discovering new Pokemon for me.
D/P were fun because it was the first time I played a Pokemon game in Japanese, my friend and I couldn't wait and we saw it at an import store, then 2 of our other friends bought it as well. It was great times too, but the overall feel wasn't as good as R/S/E.
B/W and B2/W2 did a lot of things right, but I just didn't have anyone to play or discuss the games with, so it wasn't as exciting. I barely even touched B2/W2 :(