Tau, because they evolve freakishly fast. Much like real life humans. I mean, 500 years to go from halberds and horsey men to atomic bombs and laser-guided missiles? 1,000 years ago it was all "arrgh! I'm a viking!" and Norman the Conqueror for christ's sake! Heck, even the new, inventive ways of killing each other in this century! Chemical warfare was practially invented, refined to an art form, and then banned outright. (unless you count rotting animals being thrown over castle walls NBC warfare) Cartridge weapons became mainstream, tanks were introduced, we learned to fly and almost immediately adapted that for war. Rapid fire weapons, body armor, spy satellites that can read license plates, even the art of camoflauge, were all devoloped or made practical in the space of 100 years.
When you consider the tau only need 7,000 years to go from blackpowder and castles (basically middle age tech) to being the second most advanced race in the galaxy, second only to the eldar, that's a hell of a tech leap. And seeing as how it took the imperium 10,000 years to go through three marks of SM battle armor, 32,000 years from today to phase out the assault rifle, and they're still arming their basic troops with solid slug weapons and flak armor. (Autocannons, battlecannons, IG flak armor) Plus, they show a relatively primative understanding of technology anyway. They worship computers as "machine spirts" and require guardsmen to pray before battle that their rifles won't jam.