Hmm.. that's tough. I feel like, gameplay-wise, a pay to play is the most fair. Then it feels like no one has a real advantage over someone else. I've only really gotten into a handful of MMOs, though. I played Runescape as my first, then Conquer Online, then Evony (MMORTS), and now I'm playing WoW. All except WoW were free to play, and the only reason I agreed to WoW is because my girlfriend and a few other friends were really into the game back in the day, and they wanted to play with me. Plus, now that I'm a bit older and actually have a job, I don't mind the $15/month fee, but when the game first came out, I just couldn't justify it.
With that said, I still think I prefer free to play. My gaming is so sporadic. I prefer to just have the ability to pick up the game whenever I want and play for as long as I want. I don't like that I have to pay $15 for a month when I may only play once or twice in that month. The next month, I may play 20 days out of the month, which makes it more worth it.
I never play MMOs competitively, so I don't mind the 'pay 2 win' aspect. I simply just enjoy the adventure. I enjoy seeing my character grow, and I really couldn't care less if the guy next to me just paid $50 to get to the end of the game faster than me. It doesn't really affect my enjoyment of the game. As long as the 'pay 2 win' aspect is reasonable, then I prefer that model for sure.