My bets for the opening round are:
Back Mexico 2.32
Lay Italy 2.78
Lay Nigeria 2.30
Lay Germany 2.24
Back Russia 2.38
This is the best value I have found looking at an average of the FIFA and ELO World Rankings (although I agree they are not ideal they are a decent guide).
* BACK Mexico are 38 places above Cameroon and have 43% chance (this for example I feel is better value than Chile who are 36 places above Australia and a 68% chance.
* LAY Italy who are 2 places below England and a 36% chance
* LAY Nigeria who are 2 places above Iran and a 43% chance.
* LAY Germany who are 3 places above Portugal and a 45% chance.
* BACK Russia who are 33 places above South Korea and a 42% chance.
Next best of the other lays to look at are Spain and Chile
Sorry that should be Spain and Colombia.