I'm going to nominate Piloswine for High tier.
Basically, catch one on your way out of Giant Chasm, open the path back there from Lacunosa, and fly out to the PWT. Tutor AncientPower and Ice Fang with a couple of your Heart Scales, TM Dig onto it, and throw the Lucky Egg on. Fly back to Lacunosa, resume your trip to Victory Road. One level-up and it'll evolve, getting decent bulk/speed and a 130 Attack stat; this will let it largely hold its own on the way to Victory Road, while the Lucky Egg will allow it to get to level 50 in no time at all. It'll have Earthquake fairly fast, too! You should have Mamoswine at about par with the rest of your party (just a couple levels lower) by the time you enter Victory Road; it's incredibly strong, holds its own against all the Elite Four barring Marshal (don't let it fight Reuniclus or Cofagrigus and you're good) and can gut the Champion if you have something to take care of Lapras. The downside is that, as good as it is, it comes REALLY late. You could use it during the Giant Chasm events, too, but it really wouldn't help that much; ground STAB is okay against Plasma grunts and great against Achroma, but it'd be quite a bit slower to level without the sizable gap between itself and the opponents, and it's not doing much of anything against Kyurem or Ghetsis. Only reason it's not Top is that it does come REALLY late.
Other than that:
Zorua for High. Good mixed attacker (though the lack of Special STAB is a problem) with a fantastic ability and boosted EXP.
Riolu for High. Starts slightly slow, but gets to be perfectly serviceable once it learns Force Palm, and is absolutely unstoppable after evolution. Not helping much against the first few Gyms, though. (It can murder the later Gyms even without a type advantage due to its sheer power, though. Shizui's Gym pretty much involved spamming Aura Sphere until a Jellicent showed up, switching to Zoroark, Night Slash, switching back.) Murders Team Plasma with Fighting move+Dig.
Grumpig for Low. Can be caught fully-evolved and taught Psychic from an easily-obtained TM, which is nice, but it comes late, its offenses are just all right, and it doesn't get much beyond Psychic attacks and Power Gem. Doesn't do BADLY against any of the Gyms, but doesn't do well either, and is eaten alive by Team Plasma's many, many Dark-types. It's a decent Psychic-type, but Sigilyph is better and, frankly, you don't NEED a Psychic-type that much. Great against Marshal, but that's a rather long time to be lugging this Pokémon around for one endgame fight.