Football is a team sport - it's important for the whole team to play well and do well for the United player in that team to be considered the best at the WC. Except for Mata and DDG who will barely get any playtime, we don't have any player in the 4 favorites to reach the final (Brazil, Argentina, Germany, Spain). Our players are concentrated in teams that could be described as 'dark horses' at best. If any of those 'dark horses' does very well, chances are that our player in that team will be the best United player in the WC. It's anybody's guess which of 'dark horses' will do well - England, Belgium, Holland, Japan, France, Mexico and Portugal could all do it; Ecuador are less likely.
Then there is the issue of getting enough playtime - only Rooney, RVP, Kagawa, Evra, Valencia, Nani and probably Fellaini could be considered first choice for their national team.
Obviously Rooney, RVP and Kagawa are the most popular choices, I would add Evra to that list too.
But I would bet on the darkest of all dark horses - Nani... I bet not many would expect Nani after the terrible season he had, but consider this:
1. Portugal with Ronaldo have a very good chance to progress very far - they have done very well in the last couple WCs or ECs.
2. Nani is pretty much guaranteed to play every minute unless injured.
3. He didn't play the grueling 50-60 games this year like the rest - he will be much fresher.
4. He has a good understanding with Ronaldo, so he is very likely to assist or score at least a couple goals - probably some important ones in the later stages.
5. Portugal will have much more support, than any other team except Brazil; with all the ties between Brazil and Portugal it's going to be the closest as it gets to playing at home for them.
6. Climate is going to be less of an issue for Portugal, than for most of the other European countries.
7. After the season he had he will be really hungry to prove himself.
8. Last but not least - playing alongside a great player like Ronaldo is going to help him lift his game and put some gloss on his performance.