Beholder 30
Displacer Beast 20
Gelatinous Cube 23
Githyanki 14
Gnoll 8
Lich 24
Mimic 23
Mind Flayer/Illithid 23
Owlbear 19
Purple Worm 23
Rust Monster 20
Tarrasque 5
Umber Hulk 21
Purple Worm encounters have always been memorable.
Never met a Tarrasque. Every other creature, if I saw it in an illustration, would be immediately identifiable and scream "D&D", but I am not sure a picture of a tarrasque would be for me.