far and away the most difficult tire i've ever had experience installing on a rim is the schwalbe marathon plus.

i've had two sets of marathon pluses in my life, a 20" set for a dahon folder, and a 26" set for an old MTB commuter conversion. in both cases getting the tires on to the rim for the first time took an unbelievable amount of effort.

the best thing about installing a marathon plus is that after you've done, all other tires will seem ridiculously easy to install.