It really depends what your strategy is or what your team lacks in. By Strategy, I mean Weather. If you have a Sandstorm team that your setting up, Garchomp is your best Choice seeing as it has the ability Sand Veil, but you will need a Hippowdon/Tyranitar in your team to set up the Sandstorm and sweep.
Now, by what your lacking in, I mean stats. If your team is all Attack, a SP Attacker won't hurt. Salamence is recommended if you lack a SP Attacking force, and Haxorus is recommended if you lack a Physical Attacker. Movesets need to support the strategy and stats you Dominate in. Hope this helps...
EDIT:For a Speed Boost, Haxorus or Garchomp is the way to go seeing as Salamence has good Speed. Haxorus is more recommended than Garchomp for a Speed Boost. Hope this helps...