In R/B/Y my absolute favorite is Dewgong, it's quite charming, if not the most powerful. It seems intelligent to me and I love to imagine it flowing in icy water. I'm a winter person. I have managed to get Dewgong's stats quite good maximum with Attack 234, Speed 229, Special 279 and defense 251
G/S it must be Umbreon. It's attack and special attack are not too great though.
But it's still in my party