1. Ho-Oh: This was actually my favorite Pokémon for the longest time. The biggest reason for me falling in love with it was that I beat my cousin (who I looked up to a lot when it came to games) with it. He was using Legenaries as well, mind you.
2. Mew: It's super-cute and mysterious! Ancient Mew was my first Mew card, but I think what really got me addicted to Mew was when I found a Southern Islands one as a bonus in the back of a blister pack (we didn't have the binders around here, so I didn't even know it existed). To this day, Mew is my most-collected Pokémon card -- I'm currently up to 16 variations (17 if you count the non-TCG Burger King card).
3. Raikou: It was hard to decide between it and Jirachi, but, while I'm usually a cute-over-cool kind of guy, I've just always found Raikou really cool. I've never really used it in the games, but I particularly like its Neo Revalation holo card. So majestic!