I personally love a good kroot squad. 20 strong + however many hounds hugging cover are easily able to destroy an entire chaos marine unit or similar on the charge even when you factor in whatever casualties you have sustained. 3 attacks each at Ws4 and S4 at full size (kroot + hounds) can pump out nearly 100 attacks, nearly half of which will hit on a decent combat unit or about 65 on most other units. Think of how much damage that can do. Heck, even if you lose half your models, you'll still have more than enough to take out most units. You just need to hug the cover that kroot specialise at and its very easy to make them survive long enough to hit hard.