Hmm, really depends on what you mean by best. Here's my choices for ranged and CC.
Marines- Assault Cannons, Force Weapon
Chaos- Obliterator's mutable weapon, Dread Axe
Tau- Railgun, Kroot Rifle (For cc)
Witchhunters- Faith Points
Eldar- Prism Cannon, Haywire Grenades
Necrons- Possibly the Particle Whip, Warsythe
Orks- Zzap Gun, Power Klaw
I picked these weapons because I feel they do what they do better than their counetrparts when used in the same situation and thus the list is irrespective of the strength of massed firepower. Saying that 100 Las Guns beats one Earthshaker gun is true, but no number of Las Guns are going to kill a Land Raider. On the other hand, the Earthshaker can kill troops and armor while providing a solid ability with pinning and ignoring the weak BS of the IG army as a whole. I picked the Railgun (Hammerhead type), Prism Cannon and Obliterator for the same reason, effectiveness in multiple roles, thus making them better all around weapons.
For the Orks, the Zzap gun comes out on top not because of multiple role ability, but because it fills an important gap in the army. In other words, the Earthshaker will give them good anti-armor and great troop killing ability, but since the army already has tons of troop killing ability, the greater power of the Zzap Gun against tanks makes it the better weapon for the Orks.
I picked Haywire Grenades for Eldar CC because they are the best anti-vehicle grenade in the game. Wych blades or Singing Spears could go here, but while they are great to offset the low Str of the Eldar, they are weaker against models with good armor saves, while Haywire Grenades are good against nearly every vehicle in the game.