I am taking more of an interest in WW1 these days though the Napoleonic wars, US civil war, Korea and Vietnam are also of great interest. I think Sean is right on the WW1 thing, it is rather hard to avoid it when looking at WW2 because so many aspects of the latter developed (or in come cases did not develop) from the former. One thing in particular catches my attention and that is the tactics used in various wars. The fact that so many tactics that developed so early on are still in use today, for example the 10 man section (invented by Gengis Kahn) and the 8 man section (used by the Romans). Even the tactics of fire and movement (pepperpotting and the like) that are still used today developed before WW1 and were used early on.
Almost forgot... The battle of Troy...my big favourite!! Who could forget Achilles, Hector, and finally the Odysseus´ travels across the ocean...