I'm gonna have to go against the current trend here then and put my vote towards Blackarachnia.
Airrazor is pretty and nice, and those are good qualities... but thats all she is. Pretty and nice. There's no edge, no mystery, nothing seems to irritate her all that much. As far as characters go she's a bit flat and two dimensional.
Blackarachnia on the other hand has a bit of sex appeal (for the other man-bots

), she tends to be two faced, is out only for herself and certainly isnt above using certain... talents... to her advantage.
Consider also that BA definitely has a more feminine form the Airrazor
Granted she too is two dimensional for a good long while but she eventually grows and develops as a character (Spiders Game).
Airrazor could have definitely been in a good position to upstage her if the developers had kept her on board and developed her character in some way shape or form. As it stands, within the series at least, Blackarachnia tends to win.
I realise this was meant to be about looks but personality and character also factor in. You can have the most beautiful bombshell babe and she'll still be unattractive without a personality.