Best one was the one where i didn't have to ask for it. She took my phone and put the number in herself
The worst one is a tougher one...i guess the one where she started playing the stupid "why don't YOU give me your number and i'll call you" i gave it to her (what's a guy got to lose?) but assumed i wouldn't hear from her again but funnily enough she did contact me after i gave her my number. Go figure.
Didn't quite work out though compared to the first one
Generally i just say something along the lines of "You seem like a cool person, we should talk again. Whats your number?" or "it was fun/cool/great meeting/talking you, we should do it again soon. What's your number?"