Do you mean individually, like "Alien" and "Predator"? Or AvP with both of them?
Aliens Omnibus 3 has a lot of winners, but I'm a huge fan of "Labyrinth", although "Salvation" and "Sacrifice" are great, too.
The Predator Omnibuses are harder to recommend because they can be hit or miss. Omnibus 3 is probably a good bet with "Bad Blood", "Kindred", and "Strange Roux" but the rest of the stories in it aren't as good.
As for the AvP ones there aren't a lot to choose from since there's only 2 Omnibuses, haha. The first one has a lot of winners, including the first AvP series, as well as "Blood Time", "Duel", "Eternal", and "Old Secrets".