In RBY - Snorlax followed closely by Farfetch'd.
In GSC - None really. Didn't complete the game and was kinda falling out of the franchise because I didn't like the selection of pokemon compared to RBY. It just didn't feel quite right. Miltank, Smeargle, and Misdreavus were well liked though.
In RSE - Didn't play this series, but still have favorites. Mawile and Gardevoir tie, followed by Zangoose, Spinda, and Milotic.
In DP - Started playing again. Lopunny, Mismagius, Toxicroak, and Lickilicky all kind of tie.
Notice a trend? I am a big fan of Normal types.