Most memorable - Half Dome 15.6 miles. Summer 1994. Beautiful weather, but I was not prepared for western weather (i.e. dry) and the 2 litters of water were not enough. Felt terrible for two weeks after that due to dehydration.
Most arduous - Mount Rainer Skyline Trail to Camp Muir (Base Camp) 8.6 miles. September 2004. This trail averages 1000' of elevation change per mile. Seemed like the incline was 45 degrees the entire climb! Got above treeline and I am in white out conditions, could not see the trail up the glacier (I was hiking alone) and decided to retreat. Arrived in time to have tea and cookies at the Paradise Inn (was able to get a hole in the wall room, literally!) Next morning, brilliantly clear weather, decide to attempt again, but just as a day hike (day pack) without gear and pack for an overnight at Camp Muir as was originally planned. Made it about 1000 feet below Camp Muir (I could see the hut!) with thick black clouds moving in. It took 2 hours to hike from 7500' (tree line) to 9500' and the change in the weather was concerning me. Decided to retreat once again, took about 30 minutes to back track that same distance and could have kicked myself for my lack of persistence. But this was the trip that I discovered excruciating pain in my upper back when hiking up inclines, probably the result of a motorcycle accident in 2002 that resulted in broken ribs and clavicle. Haven't really done much hiking since, but was planning a Philmont trip with the Scouts for 2021, COVID has kicked that down the road a bit!
Most Surprising - Mount Katahdin (Northern Terminus of the Appalachian Trail) 10.0 miles August 1996. I grew up in VA, so I thought the entire Appalachian Mountain range looked like what I knew from VA - lush, green, gently rolling mounds, not the shear peaks I had always associated with those in the western states. Boy was I mistaken! The Knife Edge Trail was definitely a surprise (it was fogged in, so the full effect may have been lost on me). I was on my honeymoon with my wife, so I was wearing a new pair of hikers purchased at one of our shopping stops in Maine. Couldn't walk that night due to stone bruises on the bottoms of my feet. (There seems to be a pattern on injury here!)