That question is hard to answer, because a mountain's difficulty is often associated with the particular conditions under which it is hiked, rather than the mountain itself.
The worst for me was Seymour, without a doubt, because we climbed it the beginning of August 2000; there had been a lot of rain that July, it had rained the day before we hiked, and the mud and bugs were just awful. We had difficulty finding the herd path up the steepest part of the mountain, too. Barbara McMartin's guide book said the herd path section of the route should take something like an hour forty-five minutes; it took us three hours to reach the top, and another two to get down. Plus there's the truck road in and out, so it was a 12 hour 20 minute hike for us. Another consideration on that hike is that my son was just eight years old then, so my ladyfriend, Susan, and I couldn't hike as fast as we could have if it had just been the two of us. And if I'd been alone, I could have gone even faster. I don't know if it's fair to blame the mountain for any of those factors, though.
Redfield was the one that caused Susan to break down in tears when she reached the summit, but that was in 2000 also, long before the herd path was fixed (which it was, last year) after it was badly damaged by Hurricane Floyd.
I was pretty nervous about tackling the Dix Range by myself back in September 2001, but the hike went magnificently, and it was one of my all-time favorites. One of the reasons it was so memorable was that I did the hike on September 11, 2001, oblivious to the horror that was taking place downstate. Somehow, on a couple hours sleep, I was able to climb Cliff Mountain the next day, and maybe that was my hardest, because my heart was absolutely not in that hike at all until I reached the herd path and was able to concentrate on hiking. There are some pretty tough places on the climb, particularly on the downclimbs, but the herd path went by relatively quickly, and when I got back to the trail I actually considered going up to Gray, but didn't. And it was a good thing I didn't, too, because I suffered shin splints on the long hike back to the car, so I was in a bit of pain the last six or seven miles.
Again — none of that was really the mountain's fault, except for the steep chute that had to be negotiated.
Sawteeth, we went down the Scenic Trail, but I felt pretty sick to my stomach on the climb up the Weld Trail, so the first half of that hike was pretty hard.
Dix, I felt sick —like I was going to keel over dead — most of that hike, too. Really dragging. Two days earlier on Rocky Peak Ridge, same thing during the hike out. Susan didn't even make it to the top of that one, so we need to go back (it's on the schedule to be her 46th).
Street and Nye, I couldn't bear the weight of the pack on my back any longer, and shed the thing when we reached the col. Susan, bless her, carried it much of the way for me. Come to think of it, I felt pretty crappy — at least during the last few miles — on the hikes of Big Slide, Esther, and Table Top, too.
Heck, I remember getting spooked by McMartin's description of Armstrong. It wasn't as bad as I imagined it to be, though. Neither was the descent of Gothic's cables.
So to answer your question...
I don't know. All. Or none. They all can seem difficult while you're doing them; afterward, hey! it wasn't that bad!