I'd of course like to meet them all, but if I had to narrow it down to one it would definitly be Paul. He's my favorite and like my #1 hero ever, and I would like to meet him at any time of his career, now or back in the day. Preferably the Beatle days of course, like '64 or '65!
I actually want to start thinking in advance of what to say ('cause ya just never know, but yeah right, like it'll ever happen!), but it probably won't all come out the way I want it to since I'd be very excited but nervous all at once. I'd probably just say "Thank you so much for everything, you're the greatest, and I love you, I love you, I love you!" What I would really like to do is spend a day with him in the recording studio or something, and just really talk about everything and ask all the questions. I would love to carry on a conversation for as long as I want, and I would ask him to play me a song if he had an acoustic at hand!
I think if you just met one of them face to face for a few seconds, it would maybe be a good idea to have a letter written to them in advance and say all the things you want in it. Then, if you met them real quick, you could hand it to them personally so you know there's a better chance that they'll read it.