I used to run Porygon2 but the metagame has shifted considerably and it is hardly viable these days. It was very useful for nailing Garchomp and Gliscor but it seems Knock-Off has only become more common recently.
Mega Gallade. While it's not as good as Mega Medicham nowadays, I like it because of its slightly better bulk, better Speed tier, and more reliable Fighting STAB.