History
Far too many to choose from, some great ones mentioned already (Cannae, Midway, Issus, Thermopylae). Will get back to you on that.
Fiction
Used to be Siege of Gondor (book version of events, but film for of awesome); but now it has to be the (essentially) running battle that is the Chain of Dogs in Steven Erikson's Deadhouse Gates (part of the Malazan Book of the Fallen series).
Serious respect for the Engineers in that one (the "bridge trick", and creating an artificial hill for the cavalry and so on). Plus the awesomeness of the whole concept (and the execution too).