If I remember right, I believe it was Blastoise.
Blue was my first Pokémon game, and back then, I had tendency to put most of my focus toward only a single team-member while going through main story-line, and in this case, it wound up to be Squirtle, which was my starter. After a bit of level-up, it evolved into Wartortle, and eventually to its final stage, Blastoise.
Unfortunately, I had to abandon this Pokémon eventually, as I haven't had any chance to transfer this Blatoise to future game releases. If there had been a way, I definitely would have still had it by now!