Kanto:
Charizard; His personality was just completely awesome. I loved it when he, at first, ignored Ash's commands, but then came to trust and obey him.
Pikachu: Basically same as Charizard, except that he's the MOST loyal to Ash, and he can basically do anything he sets his mind to.
Pigeot: I always loved Pigeot, I was furious at Ash when he released it. I almost stopped watching the anime.
Johto:
Chikorita/Bayleef: I loved her little crush on Ash, and how she would basically crush him after she evolved because she forgot how big she was alfter she evolved. It was a great thing to pick me up on saturday morning cartoons.
Hoenn:
None, I didn't watch the Hoenn anime.
Sinnoh:
Buizel: I love his personality. We needed a stubborn, meaner Pokemon.
Gliscor: I just saw it's first battle today, and I fell in love when it tackled Ash in glee when he beat a very tough Defensive type. Nice shows of personality.