Very late reply, but whatever.
What are your goals? Silver symbols? Gold symbols? The former can be done with solid strategy; the former needs more precise breeding and efficient use of TMs and such.
(Here's how I handle the Frontier:
https://www.pokecommunity.com/threads/319303 ...but RNG abuse isn't necessary. You can win just by breeding for pretty good IVs and good natures, but you'll want to EV train if you want to get past silver symbols.)
Anyway, the "best" team doesn't really exist. But of the Pokemon available to you, I particularly recommend:
Metagross @ Choice Band / Lum Berry
Trait: Clear Body
EVs: 4 HP / 252 Atk / 252 Spd
Adamant Nature (+Atk, -SAtk)
- Meteor Mash
- Earthquake
- Rock Slide / Hidden Power [Rock]
- Explosion / Agility
Strong, bulky, and reliable. HP Rock if you want to RNG abuse, but that's not really necessary with Rock Slide available, and it's arguably better for the flinch chance. It doesn't NEED max Speed, so you can bulk it up. Agility is usable with a Lum Berry, but Explosion is so useful and AgilityGross can be disappointingly weak. Use Return or Double-Edge over Explosion in the Palace.
Milotic @ Leftovers
Trait: Marvel Scale
EVs: 252 HP / 252 Def / 4 SAtk
Bold Nature (+Def, -Atk)
- Surf
- Ice Beam
- Toxic / Hypnosis
- Recover
Very sturdy wall Pokemon, but a pain to get. Walrein is okay, but those nasty weaknesses to Fighting and Rock aren't doing it any favors. You can run a similar build on it, but you would have to employ ChestoRest.
Heracross @ Lum Berry / Choice Band
Trait: Guts / Swarm
EVs: 4 HP / 252 Atk / 252 Spd
Jolly / Adamant Nature
- Megahorn
- Brick Break
- Rock Slide
- Swords Dance / Return
Heracross can set up on Waters and hammer them with STAB Megahorns. CB is usable--and really good in the Palace, in my experience--but overlaps with Metagross. Use Guts always with CB, but Swarm is okay if you're running Lum since you really don't want your set-up sweeper statused, but it does allow it to take burns. Up to you.
Salamence @ Lum Berry / Choice Band
Trait: Intimidate
EVs: 4 HP / 252 Atk / 252 Spd
Adamant Nature (+Atk, -SAtk)
- Hidden Power [Flying] / Fly / Aerial Ace
- Earthquake / Brick Break
- Rock Slide
- Dragon Dance / Brick Break
Only use Brick Break over EQ if you can't spare the TM. Anyway, DDMence is pretty solid, and CB hits hard right away. Double-Edge and Fire Blast are usable, but Skarmory isn't a real threat in the Frontier, and Forretress can't take your Flying attacks well. Double-Edge hits random pokes carrying Ice Punch harder than any other move--think something like Hypno or Milotic--and that's nice.
Other usable mons include Medicham (kind of an inferior Heracross though, in my opinion), Duclops, Regice, and Lati@s. Alakazam is a little frail. Donphan can be a decent physical wall.