Difficult question, I think. The "famous" ones are indeed very good - Elite, Exile. SimCity works amazingly well considering it's running on a 6502 with less than 32K of RAM.
Chuckie Egg is famous in the UK but I found it to be overrated.
Plan B and Starship Command take good advantage of the high-res 1-bit screen modes and look great.
Thrust has amazingly realistic feeling physics, very satisfying to play.
Revs is a very dry simulation of car racing, but well produced if that's your thing.
Firetrack has probably the smoothest vertical scolling on the system, which until that game came out was thought to be impossible.
Chuckie Egg is famous in the UK but I found it to be overrated.
Plan B and Starship Command take good advantage of the high-res 1-bit screen modes and look great.
Thrust has amazingly realistic feeling physics, very satisfying to play.
Revs is a very dry simulation of car racing, but well produced if that's your thing.
Firetrack has probably the smoothest vertical scolling on the system, which until that game came out was thought to be impossible.
Those are the ones I can think of right now!