Unidentified Fantastical Object
I agree with Undefined Fantastic Object, but I think Embodiment of Scarlet Devil beats Subterranean Animism in terms of difficulty.
This is a hard question. Everyone have its own opinion. Personally I think EoSD is the easiest one.
Generally, it would appear that there's level of consensus that SA seems to have the hardest patterns by deliberate design, UFO has the most risk involved (Running around to get UFOs), and PoFV/PoDD has ridiculous random bullet spam and survival required. But all the games can be hard as heck.
Subterranean Animism definitely has the hardest difficulty in terms of Normal mode, which has been compared to other games' Hard modes. I've heard UFO has the hardest Lunatic mode. Not forgetting the PC-98 games, HRtP can be hard due to the switch in genre and I've heard conflicting reports on the difficulty of Lunatic mode on SoEW.