Battle-wise, Aegislash. Loooove its typing, signature move, and the fact that it can be physical, special, or mixed. Doublade is also pretty fun to use in battle, especially on Trick Room teams! Design-wise, I am in love with Sylveon. It is definitely my fave Eeveelution design to date, and its shiny color palette is absolutely gorgeous <3 If you want to count Megas, then Megas Mawile, Absol, and Manectric. Mega Mawile is a beast in battle, plus she has a lovely design and great typing; Mega Absol's got a really nice design and Ability, and it seems like it'd make a nice Glass Cannon (I haven't gotten the chance to use one yet, sadly); and Mega Manectric is probably my fave Mega to use in battle at this time, especially given its high speed, high Special Attack, and good movepool. Plus, Mega Manectric's shiny color palette is really nice~