I think most would agree the South was the weakest region. I'm okay saying the West was the most wide open. Not sure it was the toughest. Gonzaga was the lowest rated 1 seed and Michigan was the lowest rated 2 seed. I think you could argue, however, that Texas Tech was the best 3 seed, and FSU was the best 4 seed (Auburn would be ahead of FSU if they were a 4 seed, though).
I think UNC had the toughest region, personally. Auburn was the only 4/5 seed I was worried about. Kentucky and Houston were both solid 2 and 3 seeds. Iowa State just won the Big XII Tourney - and Wofford was playing just about as well as you could expect from a 7 seed.