Of all the books, I think Engineers is the best, mostly because the group finally meets the wizard, there are do-or-die obstacles to overcome, and there are serious choices to be made. It's also interesting to see the amount of retconning Niven does over the course of the series, as the Known Space universe became fleshed out more, and the fan geeks began doing calculations on how a real Ringworld would behave. When Louis Wu first landed on the Ringworld, Niven hadn't yet written Protector, so Louis has no idea who the Engineers could be. Move forward ten years, and now both Louis and Speaker/Chmee are fairly confident in their suspicions about who the Engineers were.
My current read is The Last Best League, a book about the Cape Cod League. I'm about a third of the way through, and am enjoying it for the most part.