12 months ago it didn't look like Spain can be even touched by anyone, but they've been slowly falling apart since then. The injuries and burning out of Barca-core has taken its toll. To tell you the truth i was looking forward to see them defending the title just to have something to tell my children about. Nevertheless they will be a strong contender.
My money is on Germany, its not just the players, its the system they've worked on and refined for over 8 years now. France has fantastic squad as well. I'll also never dismiss Italy, they may not be as impressive on paper, but as long as Buffon is in goal, anything can happen. That guy is a freaking wall even in well into his 30s. Unless he gets injured again then they're screwed.
edit: also, yeah, Poland has propably the best squad in a generation and they play at home. I seriously doubt they'll win, but i wouldn't be surprised if they knock out some of the high profile sides in the play-offs.