I usually take units of 5 hounds and units of either 3 bikers w/ 2 melta (or plasma) or units of 5 with powerfist champ and 2 melta (or
plas).
One notable exception is if I take a grimoire in a daemon detachment or Be'Lakor in a detachment. In both of those cases it is usually worthwhile to take 1 large unit of 15-20 hounds to use the grimoire on or to put invisibility on. This unit is huge for winning games as they can give a huge deal of board control and sweep up entire board corners with a single good round of charges and combat resolution.
IMO hounds are the best khorne unit in the game, luckily I have amassed ~50 of them for cheap on ebay over the years. Some day I will rebase them
There are also a few tricks you can pull to further improve and buff hound units. Most of them are not really worthwhile in games less than 2000 pts though.