For me, I'd have to say Old School X-COM (UFO Defense, TFTD, X-COM Apocalypse to an extent). I love them in a way that I just do not enjoy NewCOM, but the interface was quite possibly based on the recovered works of a 13th century scribe, and is very difficult to get used to for an unfamiliar player.
Arcanum: Of Steamworks and Magick Obscura
one of the best crpgs ever made, but playing it is... difficult lol
There is like a curse for the best rpgs, where the developer they can't code for shit or something. Or they are so old that you have to download a bunch of sketchy software to get it running. Either that or it has the steepest learning combined with the worst tutorial.
One of the best indeed; ugly as fuck even for its time - and poorly optimized for its time as well - but awesome.