9. Jolene Blalock
As well as donning the Vulcan ears for Star Trek Enterprise, Jolene Blalock appeared in two episodes of Stargate SG-1 as the fiery rebel Jaffa leader Ishta. As Ishta, she led a rebellion of her fellow female Jaffa under the tyrannical rule of Moloc, a somewhat stupid Goa'uld who ordered all of his female Jaffa to be killed at birth. How are you going to breed more Jaffa, Moloc? Did nobody explain the birds and the bees to you?!
But of course it's as T'Pol that we know Blalock best, the often put-upon Vulcan living amongst humans on the Enterprise NX-01. While learning to deal with the idioms and oppressive scent of humans (and Beagle hounds), we get to see a lot of T'Pol's struggles. While she was presented as a successor to Spock (while being a fore-runner to Spock), she managed to carve out her own niche as a Vulcan character amidst Star Trek.
The two characters allowed for two very different portrayals by the same actress and they stand as a testament to Jolene's skill on screen.