his best novel in terms of characters and ideas is The Devils or the Possessed depending on your translation. Crime and Punishment is perhaps his most cohesive novel ,and easiest of the large ones to read. If you can, some of his stories are very very good, and funny too. He is one of the few modern writers to try Menippean satire, made famous by Lucian etc; basically the dead or the gods, come back to earth or are on earth making fun of the place. And one of his short novellas used stream of consciousness. Oh and The DOuble is a absolute masterpiece, a psychologically superior story to say Jekyll and Hyde which is somewhat similar and written after it.
He still is my fave writer, but it has been a while since i hav reread any of his works.