Hear me out but right now I honestly think its guild wars 2. First of all the open world maps (where you explore, quest, etc) in the new expansion are by far the most difficult solo content I have played in a mmo, probably since vanilla wow. The enemies are brutal and will easily kill you if you arent paying attention and using your dodges and there is a real danger if you pull extra baddies. The maps themselves are complicated and have a lot of verticality and its crazy just traveling around. Its completely different from the typical flat zones you see in mmos, with nonelites spread out so you're never really challenged.

Also right now the raid situation is really skewed towards the hardcore side. It reminds me of burning crusade minus all the attunements. You're only going to succeed at the raid if you a) stack the right classes that are b) specced right and wearing optimized gear and c) everyone know exactly what to do. This is really different from most other mmos I know where you just need any combination of classes as long as you have some tanks, healers, and dps. There's no casual raid mode or anything, if you arent stacking groups with enough buffs you are gonna fail hard. Also gw2 has the reputation of being a casual game but acquiring the top-end "ascended" gear requires an absolute shitload of time and effort. Its required to have at least a few key pieces of ascended gear to make a serious attempt at the raid which is a pretty sizable investment. This is especially consequential because of how volatile the meta can be as they nerf and buff stuff. The +crit gear you spent months acquiring that used to be the best might now be inferior to the +DoT damage gear so you have to make a whole new investment.

There is a lot of casual content in the game but its mostly all pre-expansion content. The expansion is really hardcore difficult. There is even one map in the expansion zone where players still can't beat the 'boss' reliably - usually this kind of thing becomes easily farmable after a few weeks once everyone knows what to do, yet it is still giving people a lot of trouble.