It's hard to decide, since I have lot of fond memories, especially of the Neo sets. If I had to pick one, it would be the Wild Growth Meganim from Neo Genesis. It was just such a good, fun card, totally underrated at the time, and I loved it. I played it for probably two years, and I was heartbroken when they finally rotated Neo Genesis out. The Team Rocket - Legendary Collection Modified Format was the best, most balanced format ever IMO. Great memories...