My ultimate favourite of all time despite not being the Pokemon I play with very much would be Eevee. I've always loved Eevee since I was a kid, I loved how versatile it could be, so now I love it for sentimentality and it helped that it was absolutely adorable too!
And because Eevee is my favourite Pokemon overall, I figured out my favourite eeveelution and it's totally Sylveon. Most people kinda discount Sylveon because so many people are like 'lol it's pink and it's a fairy' but Sylveon actually has pretty great special stats and I think that it's really good to use in battle (plus it's pink and it's a fairy <3 )
Aaaaaaaaaaaaand because I had to figure out which Pokemon I actually most enjoyed using in battle and I had to go with Banette. I love ghost types (fairy and ghost are my favourites) and Banette is so creepy and so so badass that I love it.