I'd probably do the same thing as Twi and not go unless all my friends could attend. I'd hate for any of my friends to feel left out, and it gives off the impression that I might like certain friends over others. Not going wouldn't be that big of a loss for me anyway, since I don't care much for those formal events.
If I had no choice but to choose one pony, I'd give it to Applejack. The other Mane 6 ponies wanted to go for their own self-interests, and although there's nothing wrong with that, the fact that Applejack wanted to go with the intention of helping out her family gives her the best reason for attending. Also, she was the first one to ask. It was out of pure happenstance that Applejack was there when Twilight received the tickets, so she was lucky enough to be given the first opportunity to ask. I'd say those two reasons give Applejack top priority for the ticket.