My number one has been Eevee for the longest time. I've occasionally considered it tie with Glaceon for that position, but that hasn't happened in a while.
I was different in the past, and I know Butterfree, Vaporeon, Cleffa, and Umbreon have occupied the role of my favourite Pokémon. I also know that it's possible Pichu, Mew, and Articuno have likely had that position as well, but I can't remember with 100% accuracy.
When mentioning my favs following Eevee, I tend to avoid the Eeveelutions and will list them on the side, as I like to keep things diverse, haha.