I've always found the Vietnam years to be quite interesting.

Anything from when the Romans left to when Henry Tudor came to the throne. Battles were organised tactical games then. Never again will two armies face each other over a battle field waiting for the first move to be carried out.

An interesting period is that between the Napoleonic Wars and WW1.  There was a lot of nasty fighting both in the European continent and the rest of the world (mind you, this includes the various wars in the US - Mexican War, Civil War, Spanish-American War), but other punitive and colonial conflicts as well.

It is a period, especially the latter half of the 9th century, when technological developments are making their impact, and turning wars from the massed lines firing at each other, to wars of maneouver and adjustment to the technology.

For example, during the Franco-Prussian War, while the French actually outnumbered the Prussians/German Federation in terms of artillery pieces, the French pieces were all Napoleonic muzzle loaders, while the Prussians had a number of steel breech loaders, technically superior, and much more accurate.  In more than one battle, the Prussian artillery blasted the French.  Where the two forces met in head to head battle, the French gave as good as the Prussians.  (Note that this quickie discussion does not deal with the issue of overall generalship in which it can reasonably be said that the Prussians had the advantage).

This period also introduces the breech loading rifle to the troops, first single-shots, and then the magazine-loaded rifles (the US went into the Spanish American War with single-shot breechloaders, vs the Spanish already had Mauser magazine-loaded rifles, and in some pitched battles, caused more US casualties than they received).

You have the introduction of smokeless powder (at Queen Victoria's Silver Jubilee the French battleships fired smokeless powder and amazed the rest of the assembled personages), and "weapons of mass destruction" in the form of rapid fire weapons (Gatling-type guns, etc.)

All in all, quite an interesting period.

(No, I didn't forget Gordon at Khartoum)

Those were the tactics in those days. The other side would have done the same thing when attacking. The Napoleonic era was a great one for warfare. With firing lines, cavalry and the likes. This whole era was erased with the invention of rifling as it ment that people could actually aim thier weapons at the enemy and not just point them in the general direction and hope to god you hit something.

I think that I find the war in Vietnam the most interesting (other than WWII of course).  I don't really think it has anything to do with the war itself, but the fact that I have heard a lot of first hand stories and experiences from it.  My father fought in the war, and as a result it has always interested me.

Roman/Greek, or ancient world.  This period lasted considerably longer than any of the others and included numerous wars while the other periods are confined to one war, and it's just really cool what these people, especially the Romans, could do with a lot of strong backs and only the most rudimentary knowledge of engineering.

My favorites:  anything that Julius Caesar wrote about himself and (this one is guaranteed to win a bar bet - who won the Phonecian Wars?) the Phonecian Wars, the second and third ones being eerily prescient of WW1 and WW2.

Post WWII, particularly counter-insurgency campaigns such as Malaysia, Oman, Afghanistan (in the 80s) etc.