Hardest peaks - it's all relative
A lot depends on the route you choose to climb. I personally enjoyed Isolation, but hiked it the long way (up the Crawford Path and Mizpah Cutoff, down the Mt Clinton Trail and Dry River Cutoff, camped near the Dry River, and then took the Isolation and Davis Trails to the summit). On the other hand, I hiked back down the Dry River Trail, which I hated at the end of the day due to poor footing and its numerous ups and downs.
Now that I think about it, there are some trails that I find either particularly tough or particularly monotonous, which has more to do with it than the peak they climb. Some tough trails are rewarding (for example, King Ravine to Mt Adams) whereas others, like the Dry River Trail, are just tougher than I expected it to be, with no real redeeming qualities such as good views. Of course, my likes and dislikes aren't going to be the same as someone else's. It also depends on how I felt the day I hiked a trail, and my expectations of the hike.
Trails that I have an aversion to include the Dry River Trail (which you can use to climb to Washington), the Fishin' Jimmy (too many ups and downs), the Wildcat Ridge Trail between Wildcat A and E (I liked the steep part up from Pinkham Notch, and the descent into Carter Notch, just not the in between), and the Downes Brook Trail to Mt Whiteface (endless water crossings). On the other hand, unlike many, I actually enjoy ending a hike on the Wilderness Trail after a long day, and I didn't mind the hike to Owl's Head (except for the section of the Lincoln Brook Trail between Owl's Head and 13 Falls, which is very hard to follow in a thunderstorm as it's getting dark).
The King Ravine Trail is a tough route to Mt Adams, but it was interesting, so it wasn't drudgery. The same is true for many other steep trails such as North Slide to the Tripyramids or the Flume Slide.
The first time I did Mt Zealand was drugery in wet decaying spring snow on the Twinway above Zealand Hut. The second time, in summer, where I came up from Zealand Rd, but continued on to Guyot and the Bonds, coming down the Bondcliff and Wilderness Trails to Lincoln Woods, was one of my favorite hikes, even though it was a long day. Similarly, the first time I climbed Mt Tom via the Avalon and A-Z Trails was a nightmare in spring snow (the A-Z Trail was not marked all that well for winter travel), whereas the next time, when I traversed the Willey Range, coming up the Ethan Pond Trail and down the A-Z and Avalon Trails, was a joy.
I've only done the Hancocks once (Spring snow again), and there isn't too much of choice of trails there.
The Caps Ridge Trail up Mt Jefferson was rewarding, not long, with a number of steep but interesting sections with lots of views. The Falling Waters Trail up to Lincoln and Lafayette is also interesting with lots of variety (several falls/cascades plus Shining Rock).
Actually, there are so many nice trails, it's often hard to choose. At any rate, like I said, I think it depends on your route. In most cases, you can choose a route up a peak that suits your personal preferences for trails, limiting drudgery and maximizing enjoyment. My one problem is deciding which trail to take next. I have favorites, but I also like trying different routes up the same peaks.
Other routes that I probably won't do again include Mt Tecumseh from the ski area (not hard, just monotonous),
What a nasty excuse for a trail. When I commented on the trail conditions, my husband responded with "That's nothing, you should have seen the Santanonis and Allen". He called me a "trail snob" when I was whining about wanting to go back to NH after about my 100th stab from some piece of cripplebrush . 
