It's big and it's clever
I was about 14, avidly devouring all of Niven's works. I actually read The Ringworld Engineers first, but Ringworld was still excellent.
Niven's got a can-do attitude towards progress that assumes that if you put your mind to it, you'll get whatever you want. Not too surprising from a Californian heir to a fortune I suppose, but at least his future looked like a place you could have fun, unlike Asimov's smug and over-worthy empires.
I loved his aliens too - always worked out with a decent dollop of evolutionary biology in the process. If Niven's aliens were green and had bug-eyes, that was because their environment dictated they HAD to be green with bug-eyes, dammit!
I have to admit I almost believed that a lot of what Niven said was true at the time. I certainly believed in Matt Keller eyes and spent ages trying to learn the skill. Eventually learnt to very slightly vary the dilation of my own pupils through intense concentration. This has not proved useful.
Dragonfall 5 books
Probably other, more adult things too, like Jack Vance, but these juvenile science fiction books were a revelation to me when I found them. The first time I read a book from cover to cover in one session was with a Dragonfall 5 book.