If you're going with fast Pokemon, the following work quite well;
Virizion @ Life Orb
Trait: Justified
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
~ Swords Dance
~ Leaf Blade
~ Close Combat
~ Stone Edge
Base 108 Speed, but you can use a special Virizion with Calm Mind
Simisage @ Life Orb
Trait: Glutonny
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
~ Nasty Plot
~ Giga Drain
~ Focus Blast
~ Hidden Power Ice / Hidden Power Rock
Not as good with base 101 Speed, but works in lower tiers
You can also run Pokemon like Celebi (Base 100 Speed), Shaymin/Shaymin-S (Base 100 and 125), Whimsicott trolls the living daylights out of people, provided it has Prankster and 'Quiver Dance Lilligant is deadly. Venusaur if you can get it becomes awesome if you can get Drought Ninetales or Sunny Day support due to Chlorophyll
Out of the slower Grass type Pokemon. Ferrothorn and Amoonguss are perhaps the best walls, wit Amoonguss being in lower tiers and access to Regenerator.
I included EVs on sets if you want to use them in simulators. :P