I hate to put a for certain favorite tag so I'll go down some of my favorites, I'll spare you the generic ones like Metagross and such because fucking everyone likes it.
Ditto - Goddamn adorable, unique, has a nice shiny, fun to use, and can make a very good revenge killer
Wobbuffet - Kinda cute, unique, makes people super fucking angry, fun to use, I just really love fun Pokemon
Bulbasaur - Muh nostalgia. I also just love it and it;'s design and all. I loved this little toad guy from the very moment I saw it as a kid, and it's the reason I always choose grass on my first play through.
>>37863690FUCKING BASED. Pyukumuku is also really fun to use in battles. I've been shiny chaining one for three hours and want to blow my brains out though