Ah!!! A post for me! I am a big Niven fan, have read about 95% of his work.
If you are interested in his Known Space stuff, I strongly recomend reading (in this order, preferably) ‘Tales of Known Space’, ‘World of Ptaavs’, ‘Protector’, and then the first two books of the Ringworld series - I don’t dislike the third Ringworld book as much as most people do, but it’s pretty different in tone from the earlier ones and not ‘required reading’. ‘Protector’ is really important for maximum enjoyment of ‘The Ringworld Engineers’. ‘World of Ptaavs’ is his first novel and it provides a lot of background information on the (mostly) extinct Slavers.
After you’ve gone through all those, you can take on whatever else of his you want in whatever order, the other Known Space stuff he’s written is good but doesn’t provide more necessary background info like those I listed. Be careful about buying the same stories twice, he’s got a collection called ‘Crashlander’ that has a lot of his older stories that are in ‘ToKS’ with added linking material. There is also a single volume that has ‘World of Ptaavs’, ‘A Gift from Earth’, and some other Known Space stories in it that I’d recomend if you can find it, but I can’t recall it’s name or find it on the internet.