Which pokemon will defeat the Elite Four..?

For Sidney I personally use a level 80-100 Sceptile. Use moves in this order: Leaf Blade, Leaf Blade, Dragon Claw, Leaf Blade, Dragon Claw.

For Phoebe I use Rayquaza level 80-90. Use Crunch on all of her poke'mon.
For Glacia I use a level 100 Kyogre using Thunder on every person.

For Drake I use anybody that knows Ice Beam(preferably Kyogre or Latios/Latias)
For Steven I use Kyogre level 100 using Thunder on Skarmory and Water Spout on the rest of his poke'mon.

Good Luck.

You may have to improvise, but these guys work perfectly.

First thing, we have to understand what Pokémon they have.

From this, we can arrange a party with types of attacks that can bypass the lot of three.

Here is a list of the Elite Four's Pokémon:

Sidney: Mightyena [Dark] (46), Cacturne [Grass/Dark] (48), Sharpedo [Water/Dark] (48), Absol [Dark] (49), Shiftry [Grass/Dark] (48)
Phoebe: Dusclops [Ghost] (48), Dusclops [Ghost] (51), Banette [Ghost] (49), Sableye [Dark/Ghost] (50), Banette [Ghost] (49)
Glacia: Glalie [Ice] (50), Glalie [Ice] (52), Sealeo [Ice/Water] (50), Sealeo [Ice/Water] (52), Walrein [Ice/Water] (53)
Drake: Shelgon [Dragon] (50), Altaria [Dragon/Flying] (54), Flygon [Ground/Dragon] (53), Flygon [Ground/Dragon] (53), Salamence [Dragon/Flying] (55)
Steven: Skarmory [Steel/Flying] (57), Claydol [Ground/Psychic] (55), Cradily [Rock/Grass] (56), Armaldo [Rock/Bug] (56), Aggron [Steel/Rock] (56), Metagross [Steel/Psychic] (58)

Let's use my team:
Kyogre [Water] (100): Blizzard, Surf, Thunder, Water Spout [Drizzle]
Salamence [Dragon/Flying] (93): Brick Break, Crunch, Fire Blast, Fly [Intimidate]
Regice [Ice] (100): Blizzard, Hyper Beam, Return, Strength [Clear Body]
Walrein [Ice/Water] (72): Blizzard, Sheer Cold, Strength, Surf [Thick Fat]
Heracross [Bug/Fighting] (61): Brick Break, Cut, Horn Attack, Megahorn [Swarm]
Absol [Dark] (60): Blizzard, Cut, Flamethrower, Future Sight [Pressure]

Sidney:
Mightyena: Salamence - Brick Break
Cacturne: Absol - Flamethrower
Sharpedo: Kyogre - Thunder
Absol: Salamence -Brick Break
Shiftry: Salamence - Brick Break

Phoebe:
All: Salamence - Crunch

Glacia:
All: Heracross or Salamence - Brick Break

Drake:
All: Kyogre or Regice or Walrein - Blizzard

Steven:
Skarmory: Kyogre - Thunder / Salamence - Fire Blast
Claydol: Kyogre or Walrein - Surf / Salamence - Crunch
Cradily: Salamence - Brick Break / Kyogre or Regice or Walrein - Blizzard
Armaldo: Kyogre - Surf
Metagross: Salamence - Fire Blast
Aggron: Heracross or Salamence - Brick Break

Note: This is just a suggestion to give you a good idea of what Pokémon to use and what attacks to launch.

Everyone should chill. There is no best starter pokemon and all of them can beat the elite four easily. I beat the leaugue with a lv. 64 Blaziken. I also won with lv. 64 Swampert and a lv. 64 Sceptile.

They are all even exept Swampert can beat Blaziken and Blaziken can beat sceptile and Sceptile can beat Swampert.

Get a Kyogre on level 60 or 75 or anywhere in between.
Sidney: Use Surf on everyone (trust me)
Phoebe: Use Surf on everyone
Glacia: Use Thunder
Drake: Use Water Spout or any Ice attack. On Altaria, depending on the strength of your Kyogre, you could use Thunder.
Steven:
Skarmory: Thunder Aggron: Surf Cradily: Water Spout or Sheer Cold or any other Ice move. Claydol: Surf or an Ice Move.
Metagross: Water Spout, Sheer Cold or any other move that can K.O. him. (It is really annoying when you don't instantly K.O. him.)
Armaldo: Surf, Ice moves or Thunder (depending on what level you're on)