I used my favorite party, Ezren, Val, Lem, Harsk, Meri, and Lini for my latest all legendary run. I will be brutally honest in that I used some of those charm things I had accumulated for the one with the leng spiders. That would probably be the hardest one, although I have defeated it with the same party a while back without those charms, which I don't think even existed in the game. however, there are bugs with those leng spiders and there are bugs overall in that scenario that disqualify it for hardest. The hardest one bugs withstanding is the one in five where there are three bosses that need to be defeated and the difficulty of each one is harder by +5 for each boss you've defeated (but not necessarily vanquished) to finish the scenario. That one, in deck 5, sucks. I did it without cheating, but it was pretty brutal for this party build. Ironically, much harder than any other scenario so far except for the one with the spiders.
I started up the game again recently in the past couple of weeks because I had a little more time to kill and there are things that weren't available when I had last played. The stash is new for me, as is the goblin path. The charms are really interesting, although I've only used them in one scenario (with the spiders). Overall, I'm quite happy with the game, but the bugs with the spiders is really off-putting. The other scenarios in decks five and six have been pretty easy going, although I've decided to take a rest from Karzoug in order to play my new party. The introduction of alternate characters is awesome. Currently, I'm running a regular (heroic, I think) party of Ezren, Amiri, Kyra, Meri, Seoni, and Lini. I wanted a more or less caster party but I like having a little muscle and a scout. I'm using all alternate classes, so Ezren is a scholar and such. Great synergy so far. That Ezren ability to draw both spells and arcane items is clutch, which is something I've often thought in the past. Clearly, it's such a no brainer others thought of it also.
So, the most hated scenario for me with the legendary party is Thassilonian Sins. Terrible. Second would be the Scaling Mhar Massif or whatever it's called. However, having ended up with -50+ on my check because of bugs certainly has something to do with that. Thassilonian Sins is tough because the scenario is brutal. The scaling one is tough because the scenario is brutal and the bugs make it worse. Even the last scenario, I can attest through experience, isn't as bad with this party build.