My favourite sleeper lens is an old and worn out 50mm/2 AI.
It is a remarkably good lens. I have two examples, both are very old, one has front element cleaning marks, the other has extensive fungus with a clear centre section. Despite their obvious flaws, they are very impressive. I first came across one in the 1970s, when a friend printed high contrast Tri-X photographs from one. Although the photographs didn't explore the lens's subtlety, it was clear the 50/2 was extremely sharp indeed.
By "sleeper" I assume the OP means lenses that are overlooked and relatively inexpensive. From experience I'd submit:
Yashica ML lenses, especially the 50/2 (pricewise)
Sigma 28/1.8, one of the cheapest ways of getting a bright viewfinder with a wide angle.
Kiron 28/2.8 - I sold mine but build quality and resolution were way beyond the price point.
Olympus 50/1.8
Canon FD 50/1.8 - overlooked for the 1.4 and available for pocket money
Chinon lenses
Helios 50mm
Value lenses that are no longer cheap:
The Tamron SP range
Vivitar 17mm (mirrorless cameras killed the bargains)
Fujinon lenses
Screw mount lenses, especially Pentax
Helios 85mm 1.5