Maybe I'm boring, but personally I think the good old guy-lying-on-his-back-girl-on-top is the best allaround blowjob position. I mean, I'll gladly play around with other positions for a while, or maybe even a whole session; but I'll always come back to the classic. But other than that, one I like is standing up and her on her knees. For shorter periods the 69 or the star 69 are great exciting alternatives as well.