Of the three Poke choices (not counting other), I would go with Metagross. I'll tell you why.
1) It's a Hoenn-introduced Pokemon (which the only one on your team that's not is Raichu).
2) It has one of the best type hybrids in the game (Steel/Psychic), and has great stats. There is not a single weakness which is 4x Super Effective. Also has great moves possible to learn.
3) Its base Poke (Beldum) is given to you! Sign, or what?
4) Gyarados may be great in my opinion, but I like Metagross better now that I know the differences. Plus, you already have a Water-type, which Metagross isn't. In fact, the types on your team don't have either Steel or Psychic.
5) Alakazam is a powerful Special sweeper, but it still has a major weakness. Obviously, at Lv. 100, it's way cool, but Metagross has the Steel half added as well.
6) [Ridiculous reason, but...] It looks the coolest.
It's great to complete a Pokedex! So I'll need some help...
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