Most grueling: backpacking Denali Nat'l Park, mid-July, around 8pm. We had our packs off, enjoying views of McKinley emerging from cloud cover, scoping out camp sites, looking forward to a big meal after our second long, mostly uphill day of hiking. A grizzly bear comes up from the other side of the ridge, heading straight toward us. We back off a bit and watch it tear into our packs. We move a little farther away and watch for awhile, see it get pissed off trying to open the bear resistant canisters our food was in, we move much farther away, watch some more, see our gear get spread over about a football field's worth of tundra, then finally figure we aren't going to be able to wait the thing out and don't want to be around when it tires of our stuff. So, 12-hour night hike out to the park road after a solid 10 hour day with a full pack, no food, no water, no extra clothes, nothing except the persistent fear we were being followed. About the only good news was it doesn't get too dark at night that time of year.
2nd most grueling: spending several hours with needle and thread repairing ripped up down sleeping bag with serious bear stench all over it, getting it in good enough shape for a wash machine. Man, I'll never forget that headache. Took five washes to get the smell reasonable.
FWIW, the bear eventually got into our food and ate everything. Ate our toothpaste, even sampled my wallet, I still have it and a credit card with a couple of holes in it. Didn't touch some carrots we had, though.
Human-Bear encounters in Denali are taken pretty seriously. Special bear tech rangers went up, retrieved our gear and tagged the bear for future tracking. It was funny, they were not pleased we left our packs, they don't like bears to figure out humans carry food. The rangers said we should of grabbed the packs, and then walked off. Let me tell you, that idea never even ocurred to me when I was twenty feet from the bear, with my pack half way between us, and the bear obviously smelling something in the pack it wanted to check out. If it did occur to me, it would've seemed to me the very height of foolishness, positively a Darwin Award type of thing to do.