Out of everybody you listed I think this would be the best and most likely match.
I think Undertaker is getting too old to put on an enjoyable match with the Rock who is still in great shape.
I would like to see this, but I don't think it will happen. Brock only has 2 matches left on his contract. He already is fighting HHH at SummerSlam and with the news that he has been in contact with Goldberg, I think that will be his opponent at WrestleMania.
This would be ok, but with Randy's 2nd violation I don't think VKM would put him in that big of a match up in the near future.
I like Christian but I don't see it happening.
The Rock vs Rey Mysterio Jr
Too beat up, too old, knees are too fucked up, and too small to make this an enjoyable match.
The Rock vs Dolph Ziggler
I don't see him as a big enough star to be in that kind of match. In my opinion Dolph is a upper mid card wrestler at best.
I like Cody more than Dolph, but I feel the same about him.
I would like to see this. Sheamus is a big powerful dude that could hang with the Rock. But I think Sheamus should make the turn back to a super heel.
I also think that Rock vs Cena 2 has a really big chance of happening, even more than a Rock vs Sheamus match.