The wittiest reply I have ever had was valentines day many many years ago. It's a long story but I was on the tube and there was a girl sat opposite me who I found really attractive - she was quite ordinary really but there was something about her.
Anyway I was a shy naive teen, no idea what to do or say, and all I could do was keep looking at her now and then. She stayed opposite me all the way home. a good half hour or so, and by the time we had got to the station before mine, we were alone in the carriage. kept thinking I have to say something before I got off, and then fate took a hand, because in uncrossing her legs, she accidentally kicked me. "Sorry" she said, to which I answered (bearing in mind the day) "do you know, if I'd have known you were going to be on here tonight I'd have brought a bunch of flowers and a box of chocolates". She looked straight back at me, smiled, and said "and if I'd have known you were going to be on here I'd have cufking walked!!".
I didn't know whether to laugh or cry when I got off, but I had to give her credit for the comeback.