Four #1 seeds are still alive. As well as a #13, #11, #7, and #6. And with four #1 seeds still alive the possibility of all four making the Final Four is very much in play. Using average seeding of the three remaining teams in each of the top seed's region here is an idea of who has the toughest bracket left.
Washington, DC: #1 UConn, #5 Washington, #7 Wichita St. #11 George Mason.
Average Seed of Other Three Teams: 7.6
Atlanta: #1 Duke, #2 Texas, #4 LSU, #6 West Virginia
Average Seed of Other Three Teams: 4
Oakland: #1 Memphis, #2 UCLA, #3 Gonzaga, #13 Bradley
Average Seed of Other Three Teams: 6
Minneapolis: #1 Villanova, #3 Florida, #4 Boston College, #7 Georgetown
Average Seed of Other Three Teams: 4.6
So based on average seeding Duke technically has the tougher potential road followed by Villanova, Memphis, and UConn. The problem with seeding is it does not tell the whole story. Boston College should have been a #2 seed so Villanova is getting a tougher game with its four seed than say Duke with the four seed LSU. The highest seed UConn will face in the regional is against #5 Washington. Since Wichita St and George Mason are #7 and #11 which means UConn could go to Indianoplis having failed to see a top four seed(Much like UNC last season which played #16, #9, #10, and #6 on the way to St. Louis) Duke and Memphis clearly have the toughest road because they potentially could see a #2 seed in the Elite Eight.
Now does any of this really matter now? I would contend it does because with each move to a new site and new "four team tournament" as Dean Smith used to call them, the pressure is greater and the momentum changes. These teams were seeded where they were for a reason and with the exception of BC, I think it is indicative of how good they are. The first four days of tournament play is always replete with teams playing above their seeding and this year was no exception. The second four days of the tournament usually results in many teams "returning to their seeding" In other words George Mason will play like the 11 seed they really are or Bradley will be overmatched by Memphis in Oakland. There are exceptions to this like in 2000 when the Final Four had a #1, #4, and two #8 seeds. The point it that the second weekend of the NCAA Tournament has a way of exposing the lower seeds. Partly because they are unable to sneak up on someone and partly because four #1 seeded coaches have four/five days to study film and break down their opponent. Whereas seedings seem to go out the window in the first and second round, the Sweet Sixteen and Elite Eight usually brings them back to reality.
That being said, I have never seen an NCAA Tournament with this kind of parity from top to bottom. I think Bradley and George Mason enjoy better odds this year than any low seed before them.