DA ORKS IZ DA BES FER FIGHTIN' AN' WINNIN'
Of all the races in Warhammer 40000, the Orks are the ones I root for the most.
The Eldar are supremely arrogant and cannot accept responsibility for their role in creating Slaanesh, nor can they accept that their time has long since passed.
The Necrons are intent on either wiping out and harvesting other races, or putting them in big collections like Trazyn the infinite. And they can't stop fighting each other over petty dynasty struggles millions of years old.
The Tyranids are an individual mind that only exists to feed, and they might be the harbingers of something even worse.
The Dark Eldar took to piracy and sadomasochism to hide from Slaanesh.
Humanity birthed three of the Chaos Gods, wallow in ignorance, and have routinely been responsible for the largest wars in galactic history. It's either mass slaughter or a police state.
The Tau are naive to the dangers of the galaxy at large, and they are prone to going rogue without the leadership of the Ethereals. See Farsight's enclaves for renegade Tau forces.
But the Orks? Conflict is their way of life. The strong get stronger, the weak get crushed underfoot, and there is no ill will. Orks don't hold grudges, if they lost it was because the stronger being won. It is a sign of respect that the Iron Ork continues his fight with Commissar Yarrick, the only humie strong enough to take him in a fight. They love war and winning, so any fight is a good fight. In essence, they are about the purest form of life in the galaxy, the physical manifestation of combat, not subject to the whims of manipulative gods or obstructive bearaucrats.