Okay, I'm a big Niven fan....
My favorite thing is that his books always seem to be based on an interesting "hard" science fiction premise, and logically build from there, but still goes into the character aspects, as well.
I love Ringworld. The cultural concepts of the Puppeteers is fascinating, to me. I, too, found some of the later in the series less than compelling.
My favorite Niven books are 2 of the mentioned that he co-wrote with Pournelle-- The Mote in God's Eye (which I consider one of the 10 must-read SF classics), and Lucifer's Hammer. Strangely, I tried reading Pournelle's solo books, convinced that I would find another author I loved, and really didn't like them at all. One of those cases when a collaboration works greater than the sum of it's parts, I guess.
I'll read anything I can run across by Niven, and re-read even my least favorites (well, except for 1, and I won't mention which, because it might be somebody else's cup of tea.)