David Gemmell and Bernard Cornwell are both well-known (in certain circles at least) for their excellent battle scenes. Simon Scarrow also does an excellent job with combat scenes. Robert Low is actually a bit too gorey for my tastes, but he has some excellent battles at sea and on land.
Scarrow, Cornwell, and Low all write historical fiction and are therefore more realistic. They also have a good mix of individual combat and full-scale battles between armies. Low writes about Viking warfare. Scarrow writes about the Roman Legions in ancient Britain and the Middle-East as well as novels following the lives of Arthur Wellington and Napoleon Bonaparte. Bernard Cornwell has written books about the American Civil War, the Napoleonic Wars, Arthurian Britain, Alfred the Great's Britain, The Hundred Years War...Well, you get the idea. He also has excerpts available on his site here: http://www.bernardcornwell.net/
Naomi Novik also writes good action scenes. She writes about a draconic air force during the Napoleonic Wars, but there are many good naval and ground fights as well.
Conn Iggulden is good too. He's written a tetralogy retelling Julius Caesar, and is currently writing a series about Genghis Khan.
Lee Child's Jack Reacher series is very good. He can make any scene suspenseful. His writing style is somewhat unique as well. The series is about a former MP living off the radar, but usually getting involved in trouble of some sort in every town he passes through.
I can think of so many more...Brent Weeks, Rafael Sabatini, Jim Butcher, Lois McMaster Bujold....