Best reaction was with my wife, actually. We'd already been married a few years, and had been sexually active while dating before that.
After a night of drinking one night, we came home and got a little frisky. I asked her to blow me, and she got down on her knees but then just gave me a handjob for a minute or two. I asked "Is something wrong?"
She looked up at me with her big (slightly drunk) eyes and says, "... You're just really fucking big," with the most sincere voice. Goes to show you can still be surprised by your wife (and surprise her too, it seems!)