Option 1: Metagross
So it's not "fast" at base speed 70, but it's got really good bulk and I like running an Agility set.
Metagross @ Expert Belt/ Leftovers
Ability: Clear Body
EVs: 196 HP, 252 Atk, 60 Speed
-Agility
-Ice Punch
-Meteor Mash
-Zen Headbutt
Option 2: Haxorus
Get that shiny one in the Nature Reserve. It's particularly good because you can soft reset for the correct ability/ nature/ good IVs.
Haxorus @ Lum Berry
Ability: Mold Breaker
EVs: 252 Atk, 252 Speed, 4 HP
-Dragon Dance
-Dragon Claw/ Outrage
-Earthquake
-Shadow Claw/ Low Kick
Option 3: Gengar
Good speed, high Sp. Atk and useful typing. Downfall is that it's not very useful in stalling anything due to poor defences.
Gengar @ Expert Belt/ Black Sludge
Ability: Levitate
EVs: 252 Sp. Atk, 252 Speed, 4 HP
-Shadow Ball
-Focus Blast
-Thunderbolt
-Disable/ Pain Split
Option 4: Starmie
The great benefit of having an instant recovery move, it also has really good speed (outspeeding Gengar) and good Sp. Atk.
Starmie @ Leftovers
Ability: Natural Cure
EVs: 252 Sp. Atk, 252 Speed, 4 HP
-Recover
-Surf/ Hydro Pump/ Scald (this depends on your team. Surf for anything with Storm Drain/ Water Absorb, Scald if you want the accuracy, or Hydro Pump if you want the power)
-Psyshock
-Ice Beam