+1 monmusu quest.
also paradox may be the vastest game in old rpgm engine?
Dungeon of Lulu Farea is a puzzle-rpg, with idiotic story, but well aware of that and actually well written and well-translated. By all means play the old WolfRPG version (fan-translated), because the official rpgm-port is awful and has 20 or so % of content (text) cut. Even on "normal" it is difficult, though.
not "unique", but still:
Ariadne has some very good dungeon maps/designs (large world, long game, somewhat difficult)
Rune's pharmacy has some very pretty maps and additional to CG scenes also little pixel-art for losing battles. Not difficult, but sorta-grindy for alchemy ingredients.
Nursery Slime for surprising me with unexpected quality. It is just overall enjoyable, and I even liked some of the sex scenes (it is not often that I do)
Eiyuu Senki - World Conquest
Lots and lots of funny silly characters (girls), much good voice-acting. Very long game, battle mechanics of its own. Sex scenes - ah, well, everyone is a virgin all the time and that's really not my thing, but I still enjoyed the silly characters enough. I recommend the fan-translation over the official, it has some extras imported from playstation versions, including translation for all the battle-, worldmap-, listing- comments - and those often are hilarious, and always add to the characters.
Do not go for Eiyu Senki GOLD, that game clearly addresses players who have already finished World Conquest and should not be played first.