Just one? That's a near impossible decision given the myriad of obsessions I've had over the years.
I mean, if it was "any pornstar any time" there are probably a dozen or more candidates that I'm not sure I could pick from. Jenna Jameson circa 1996, Briana Banks circa 2001, Gemma Massey circa 2009 just to name some favourites.
As for active right now.... Christ....
When I think about all the girls I really love watching, and how badly I'd want to experience them myself, the name I keep coming back to is Christy Mack. She's pretty much my idea of perfect right now. Although she is run close by Jasmine James, and if she counts as a "pornstar" the wonderful Mandy Flores.