Brazil to be top seems like the obvious choice. Brazil always have a very strong squad but this year Brazil are much stronger for two reason, one - they're on home soil, playing where they're accustomed to, and two - the team is better, with the likes of Neymar, Hulk and David Luiz to name a select few. It is hard to see past them in Group A.
Mexico, being a Central American team, will have a huge advantage over European sides in the world cup. The conditions (like Brazil) is what their players will be used to. Unlike Brazil, most of their squad (17 of 23) play their football in Mexico. Of the known players (to Europeans), in their squad Javier Hernandez of Manchester United will be the danger man of the Mexicans,
Croatia, on paper, are a fantastic side, with the likes if Modric, Jelavic and Mandzukic in their squad. They were as high as 4th in the world rankings in June 2013. However, I saw them play Scotland twice, and get beat twice, they were not very good in either game and Scotland deserved the victories, probably by a few more goals than they actually won by, they did not look like a side full of world-class players. That combined with the temperatures I cannot see them qualifying for the Last 16.
Cameroon, have quality in the forms of Eto'o and Alex Song but lack strength in depth. They are not the best team in Africa and are nowhere near the side of just over a decade ago. Cameroon could produce an upset but I doubt they will against the quality of the other three sides.