It gets interesting if you compare the latest UEFA rankings with the latest FIFA ones which are very similar (about 95% correlation) but not identical. Highlights include Portugal, who are ranked 11th by UEFA, but are the 5th best European team according to FIFA The table below summarises the Euro 2012 groups with each team's UEFA and FIFA ranking (where I've filtered out all the non-UEFA teams). (Click for a full-resolution version.)
Based on FIFA rankings, group B is indeed the deadliest, with an average country ranking of 4.5 (and, interestingly, every team a higher rank than any in group A). According to UEFA, however, it's not group B but group C - home to the Republic of Ireland - which is the hardest, although there is very little to choose between the two.