As much as I like sci fi as a genre, ad reading in general, outside of the Star Trek and Star Wars books I've barely read any sci fi series. While i've found a few good ones, I generally don't like taking risks on wasting my time and most sci fi not connected to pre existing franchises just don't usually seem interesting enough for me to try. I'd like to read more, but it seems like half the "sci fi" books I find on best of lists are usually realy bizarre premises that don't even involve space or spaceships (which is a must for me, I don't care about time travel or sci fi thats more earth based) and the other half just sound stupid.
That said, two that I do like (although I don't know how huge and epic they are) are the Starship
series by Mike Resnick (5 books) and the Troy Rising
series by John Ringo (3 books).
The Starship series follows a man named Wilson Cole, a former captain of the republic navy who was demoted for reckless behavior. He gets reassigned as second (or third, I forget which) officer on the strship Teddy Roosevelt, an old beat up ship far from the front lines. Eventually, things happen that lead to him assuming command, getting into a confrotation with the Republic (which is more than a little corrpt) and having the Teddy Roosevlt go off on its on. Its a very entertaining series, with a great cast of characters who I really like. I guess its technically miltary sci fi.
The Troy Rising series deals with Earth's first contact with aliens, who take over. There are a few main characters over the course of the books (although a former sci fi writer named Tyler vernon is the main character for the first book and some of the second), and it follows them and the world from being invaded to fighting back and eventually becoming a power in space. It takes a few missteps and I found the last fort of the third book kind of confusing. The ending asn't really a cliffhanger, but it does't feellike you get closure with he characters or univese. Still, I enjoyed most of the series nd consider it a good read.
Neither of these are hard sci fi, and may not fit the strict definition of "epic" but I'd still say they're both very good series and someone who likes sci fi will probably enjoy them.