Base set because it was the first set ever made and most of that sets cardsell high. The Base 2 cards were remakes of the Base set so there was not as much value to have in those cards but they were fun to collect anyway.
1sr edition base Charizard goes for $275-$325. That was a few years ago but I am not sure what it is now. There are a few others with that price range. Probably Blastoise and Venusaur and also the Base Zapdos is very high because it is a very good card. I wish I had one.