Some good choices but your order is WAAAY off. Let me clue you in:
1. Jeri Ryan - This isn't even close. 7 of 9 by a lightyear.
2. Jolene Blalock - She really rocks the pointy ears.
3. Natasha Henstridge - Hot, and into REALLY rough sex.
4. Zoë Saldana - Ten feet tall and blue. Very Nice.
5. Rachel Nichols (Gaila, Star Trek) - Less than ten feet and green. Nice.
6. Natalie Portman - You really placed her after a one-eyed, purple haired cartoon? Really?
7. Milla Jovovich - Lifetime achievement in badass babedom.
8. Sean Young (Blade Runner) - Is a replicant an "alien"? I think so.
9. Tricia Helfer - No argument. She is hot.
10. Tahnee Welch - It's all in the genes.
Honorable Mentions: Carrie Fisher, Kelly LeBrock (Weird Science, again stretching the definition of alien), Kirstie Alley (Star Trek II: The Wrath of Khan)