I vote for Temple of the Elemental Evil.

ROLFMAO

Agreed.
Special mention to Eador:genesis.

This would be my pick as well.
I went through Tyrian quite easily and without any serious hassle, I also managed to finish Raptor with not too bad perforamnce after I discovered I can use mouse to control it. Yet so far, after several attempts, I wasn't able to get past 1(!) level of Stargunner and I doubt I ever will. I don't understand how people can complete it.

Stargunner? That wasn't so hard, at least the episode one. Maybe you should invest in better engines at the start to be fast enough to dodge bullets and other bad things.

Damn it! Five hours too late! ... Though I'd imagine it's more of a limp noodle in his case. =P

^^ This and Jagged Alliance 2, for SOTS, especially if you upgrade to The Gold Edition DLC, it is so hard and tiny mistakes will kill you or bad RNG in such cases. Jagged Alliance 2 are notorious for its steep learning curve and unforgiving consequences if you even made the slightest mistakes.

I just started playing my copy. I can see where I can easily run into difficulty later.

Roguelikes are the hardest genre out there, with shmups a close second.

I did some research once by savescumming my way through ZAngband, carefully shaping my character to have the most beneficial outcomes I could, and still reached a point where I was massacred in moments by a deep-dwelling enemy. That folks complete these games, and do so properly, astounds me. And I'm saying this as a veteran rogueliker myself.

The second half of Lionheart with a diplomatic character(e.g. my first playthrough). The first half wasn't exactly easy either, but holy shit that difficulty spike murdered my character, and I just had to give up and start anew.

I once managed to finish that game on easy difficulty with a marine, then I tried "normal" and got slaughtered, then I switched back to easy, but still haven't been able to beat it for the second time. So yeah, it's probably one of the hardest games here.

The fun thing about Wizardry 7 though is that the non-linearity works out well. Sure, there are lots of places where you need a particular item from another area to move forward, but it never felt tedious to me. There are some pretty silly puzzles, though, like the bean puzzle or the entire Funhouse, but it's nothing a bit of perseverance can't solve. And even if you take forever to muddle through everything and the NPCs steal the maps, all is not lost. Since the NPCs tend to like to kill one another, the longer you take the more concentrated the maps get, so eventually you get to a one-stop shop for missing maps.

Admittedly, I did use a walkthrough for some stickier areas like the Funhouse, or "what is this Serpent Staff for again?" but I never really felt like it negatively affected my experience, and that was back in the 90s when the game was relatively new. I think the real stumbling block for new players is the character creation system. You can create characters that are simply WAYYY weaker than others, but it's often not apparent how weak they are until you've put a bunch of hours into the game. It's only once you figure out how the combat system works that you realize how important those early decisions are, which is kinda odd since you have to make those decisions before you actually know anything about the game. Class-switching can help, but it can be a long road. It's one of my top 3 games of all time, but Wiz 7 is... not very accessible to modern audiences, which is a shame.

Exactly! This game requires quite a bit of meta knowledge for smooth sailing, I don't know a single person who just started this with no prior knowledge and no help and finished it on their first blind run. Maybe someone with such persistence and willpower exists out there but it would be a surprise. That person would truly be a...Dark Savant (lol). The thing with almost requiring meta knowledge wasn't anything uncommon back then, it was completely normal and expected that people had to buy guide books or call a hotline and gaming magazines were often bought just because there were printed walkthroughs, hints and cheats in the back section of each magazine.
I played dozens and dozens of hours and got quite far into the game with what I thought was a cool party, only to slowly realize that without switching classes and grinding for who knows how many hours more, I'd most likely run into problems nearing endgame. At one point, my hand written notes for the game were thicker than my class notes for history class. I never got angry at the game though, loved it too much for that and wasn't as jaded yet but there were too many other great games and spending so much time on one single game wasn't justifiable anymore. I'd love to tackle Wizardry 7 again, maybe this time with a walkthrough. Unlikely to happen though, considering I have 10 times as many games to play as back in the early 90s.

Woot? JA2 is one of the most forgiving, assuming you adapt to circumstances. Half your forces killed? Retreat, regroup and attack again. Tank popped up and no way to kill it? Retreat and get some more explosives. Enemy forces too numerous to deal with? Bypass them and continue to next sector.

As soon as you realize that losing a merc is part of the game, you see that JA2 is quite forgiving. You will lose mercs, but that's what the mercs are there for.
And all you need to do to win is kill the Queen, something that can be done in less than 10 minutes ;)