the 2 toughest groups as I see it are group D and group G.
group G is the obvious choice as toughest with Brazil, Portugal, Ivory Coast and their fodder, N. Korea. Brazil is loaded with star talent. Portugal features 2008 FIFA player of the year Christiano Ronaldo. Ivory Coast is lead by EPL golden boot winner, Didier Drogba, who plays for Chelsea at the club level.
group C, i think, is being overlooked by many. Germany, Serbia, Ghana and Australia compose the teams in this group. Germany are always favorites to make a deep run. Serbia, who had a great qualifying run, is loaded with players playing with top clubs throughout Europe and have a stout defense. Ghana, playing on their home continent, had a nice run in 2006 in Germany and will be hoping for the return of one of the top defensive mids in the game, Michael Essien. Australia also made it out the group in 2006 and are led by Everton striker, Tim Cahill. if the US makes it out of their group they will have a tough task facing one of these 4 teams in the round of 16.