ToME: the Tales of Maj'Eyal

sCthAngband is hard because you only gain hp by being damaged by monsters, so it's difficult to get far with mages or low hp races. maybe other 'bands are worse, i did not play that many different ones (i don't know sangband for example). but as to skill system scthangband is the one i liked best so...

in my opinion CthAngband is one of the hardest, although not listed above