I'd say that the best of his is the Empire series - though that's co-written with Janny Wurts.
Daughter of the Empire
Servant of the Empire
Mistress of the Empire
The Riftwar Saga is his next best, but you say yu've read that. The quality isn't really maintained after that, but the Serpentwar saga is pretty good as well. Then there's the Conclave of Shadows series, but it's nothing special. Below that all of his work is far below that of the Riftwar saga, and only worth reading if you're a devoted fan (like me - I read them all because he was the first proper fantasy author other than Tolkien that I had read). You may as well not bother with the Krondor books, or Prince of the Blood or The King's Bucaneer, or with the Legends of the Riftwar series unless you really want to find out more about Midkemia and Feist's overall story, because they aren't particularly well written.