Kroot = Win!
Kroot are my second/third (depending if you include Mandrakes in as DE or not) race with first being Dark Eldar and the second if you don't count them as dark eldar being Mandrakes (the shadow people with glowing runes allover there bodies)
I have come to the conclusion that Skraat is not a simple Kroot. His forefathers must have fed exclusive of Wraithlords, Grey Knights, Warbosses, Baneblades and freaking Imperial Titans, throwing in some Avatars of Khaine to spice things a bit.
There is no other explanation for so much dna-fueled baddassery in a single critter.