For one thing it pretty short and is a fairly easy read. Ringworld is one of the first scifi books I ever read I thought it was amazing the first time I read it. Not as a good when I reread it as an adult but I still love the concepts in the book even if the characters are a little lacking in depth.

The only one of the sequels I have read was Ringworld Engineers and I remember liking it a lot when I first read it 30 or so years ago. I prefer to read Niven when he writes with Pournelle and I loved The Mote In God's Eye books. I also really liked Nivens Integral Tree book and it probably better written the Ringworld. Legacy of Heorot is also worth reading.

Of course who wouldn't love a writer who used to advice Ronald Reagan