First, third and Rondo of Blood for sure.
Simon's Quest is more cheap and tedious than difficult, SCIV is too easy except for a few spots, same for Bloodlines.
Ctrl+F
No Castlevania 3
So therefore, Castlevania 3. The ENG difficulty drove me nuts but it's the hardest game that I ever had the pleasure of enjoying.
Original Castlevania was hard. CV3 and Rondo? Not as much.
My entry: