The SPARTAN-IIs of Halo probably win this thread. Simply because they are trained from the age of six. No other group in that poll, or any that I can think of save the real Spartans, has ever done that. Not to mention the fact that SPARTAN-IIs are resistant to the Flood, which would wipe out every other badass faction out there, and have armour that allows them to connect directly through a neural lace to most computer systems. Add in the augmentations that make them the Halo equivalent of Wolverine, without the extending claws (rectified by stealing an energy sword) or the healing factor (rectified in the books with 'biofoam'), and the SPARTAN-IIs are the most badass group to exist.
Also, take the SPARTAN-Is like Sgt. Johnson into consideration. Johnson, as a Spartan, was still kicking rebel and Covenant ass well into his eighties. And the SPARTAN-IIIs were literally sent out on suicide missions to basically do maximum damage to the Covenant, to the point where it was solely down to them that the fleet was so small that ended up gathering at the Unyielding Heirophant (true, Master Chief decimated them after that and thus managed to drastically reduce the enemy that attacked Earth in Halo 2, but if the SPARTAN-IIIs hadn't done their bit MC's job would have been literally a few hundred times more difficult...).
TL; DR: The SPARTANs kick major space ass...
