It depends on what I want for my Mega. I'm only going to have one shiny is what I'd decided. If I choose any of them other than Sceptile, I can run with the Serperior.

Honestly for my Mega it's either... Gallade, Gengar, Altaria, Scizor, Garchomp, Tyrannitar, or Sceptile.

Also I know Starters like Charmander, Torchic and Piplup have an exclusive status within the games, but in the anime they're encountered almost like normal Pokemon, so I don't think starters have to be very rare.