I'd second Dune, if you can manage them try all of them in the main sequence from Dune to Sandworms (the last two were writtien by the original author's son and another author apparently based on the original outline/notes).
Nights Dawn Trillogy by Peter F Hamilton is very good, pretty epic (3000+ pages spread across 3 books, with from memory 5 main groups of characters*)
Eddings, again, although some of his stories seem very similar to each other.
Discworld definitely, although it's not so much a single epic as multiple stories/story arcs (Rincewind, Wizards, Death, Witches, Watch, Finance/Tech), with a cast of literally thousands (there are hundreds of recurring characters spread out across the series).
Possibly Terry Goodkinds Sword of Truth series, although it seemed to get very samey after about book 5 or 6 (it's 11 books long) - I ended up buying/reading all of them, but I don't know if I would do it again.
Asimov's R Daniell/Bailey series, start off with Caves of Steel, then the sequels, then move on to the Foundation series.
I can always recommend his Robot series of books, look for the Complete Robot to get them all in one volume (all his shorts at any rate), so many sci-fi/robot stories these days seem to have at least partial roots in those stories.
*Each of whom see the story from different angles as it starts up and gets running.