Dune holds up way better if you pretend it's just one book (the first one). It stands alone very well and IMO is completely worthy of its status in the classic sci-fi pantheon.
On the other hand, I finished TBP, but I disliked it enough that I didn't go back to the other two volumes. What makes them better than the first, in your opinion? (purely for my own curiosity re: whether I should go back and read them)