I really liked Ringworld. Ringworld Engineers was decent too, but the later ones didn't grab me.
Niven is a fantastic short story writer and I enjoyed his "known space" and other stories. His short stories on the effect on society on instant teleportation were very fun.
He also did some intriguing fantasy as well. "Not Long Before The End" and "What Good Is A Glass Dagger" about how magic was being used up in the world.
But my favourite of his books is his "Mote In God's Eye" with Jerry Pournelle.