Even though the first two Alien films are the ones that most people agree on being the best, I like the Xenomorphs themselves equally, no matter what film/game they're from, and that's because of their designs. My top 5 for the Xenomorphs' designs goes:

1 - Aliens Xenomorph Queen

2 - Alien Xenomorph

3 - Aliens Warrior Xenomorph

4 - Alien Isolation Xenomorph

5 - Alien Vs Predator Warrior Xenomorph