Personally, the top choices for me are:
Sting vs. Undertaker
Undertaker vs. John Cena
Stone Cold vs. CM Punk.
When it looked as though Sting was heading to WWE last year, rumors of him facing Taker at WM started like wildfire and people were excited. Sting is the biggest star of the past 40 years in American wrestling to have never worked for the McMahons. In terms of overall respect among both insiders and wrestling fans, Sting & Taker are both on similar levels. They're two revered legends that people can just sense would go out there and put on a great match that'd tell an incredible story. It'd be something that, in all honesty, could possibly have a once in a lifetime feel to it. Sting is well into his 50s right now, Taker is 47 and both simply don't have all that much time left. With Taker basically being used as a special attraction for WM, odds are that a match between them would be the one & only time we'd ever see them wrestle against each other. That in and of itself would make the match worth seeing.
As for Taker vs. Cena, it just seems like a very logical choice. John Cena has done everything that can be done. He's beaten everybody, he's won 12 World Championships, he's won the Royal Rumble, MITB, he's headlined every major show since the mid 2000s, he's the face of the company. He's done everything except try to end Taker's streak. Taker's streak is the one real mountain left in WWE for John Cena to conquer. As with most of Taker's matches at WM for many years now, this would have an epic feel to it and it'd be the kind of match that people would be talking about for a very long time.
For Stone Cold vs. CM Punk, I think a lot of people would view it as a passing the torch kind of situation. Stone Cold Steve Austin hasn't wrestled a match for a decade. Because he retired due to his nagging injuries, and was still relatively young when he did so, a lot of people felt that there was still potentially so much more that Austin could have done. Austin has never really had that match against a younger guy in which you just got a feeling that something significant happened in the sense that there was a changing of the guard. I'm not sure how well Austin can go, but I think Punk could carry him through a solid match that would be highly anticipated, have an epic feeling to it and would have people talking.
As for which I'd want to see the most...I'm not really sure, I'll have to think on it some more.