Depends on what's your preference, really. There's no "right" answer. If you dig chronology, of course you'd go the "Legends of Dune" trilogy, the "Prelude to Dune" trilogy, then Dune 1-6. If you aren't interested in the Jihad except as a sidestory, maybe do the "Prelude to Dune" trilogy and then Dune 1-6, the "Legends of Dune" trilogy as an afterthought. Otherwise, if you want to take full scope of Frank Herbert's work first and foremost, of course you'd want to go Dune 1-6, then everything else. I wouldn't worry too much about the Dune Encyclopedia. It's not official, it's been denounced by the Herbert Limited Partnership, and only goes from Dune - God Emperor of Dune anyway. You'll just confuse yourself when you start reading the prequels/Legends/Dune7-8. Read it if you want, but take it with a grain of salt.