So far there’s only been one person who’s chosen the Spanish-American War; if you happen to have it handy, the Naval Museum in Philadelphia has very interesting information about that one.
Does French and Indian War/Seven Years War count? That’s really my specialty. It’s what led to the Revolution. Plus, George Washington was definitely guilty of war crimes. RIP Jumonville. 
WWII. It’s the oldest one that you can still find someone alive who actually participated, but that’s probably going to change pretty soon; the average age of a living WWII veteran is in the low 90s.
WWII by a long shot. It’s not that I think the war itself is more interesting pound-for-pound, but there’s so much of it that you can hardly turn around without stumbling over some new fascinating story or exploit.
First or second? I assume second.
But even the second ended in 1902. That was before planes and tanks existed.
Generally speaking, the last war veteran dies about ~90 years after the conflict ends. The last civil war veterans died in the 1950s and the last WW1 veterans died in 2012
So the last WW2 veterans will probably die in the 2030s.
I checked both the Civil War and World War II. I was a child during Vietnam; perhaps the two wars I am more interested in offer at least some sort of the moral clarity that was lacking in Vietnam?
My Dad served in WWII (infantry in ETO). Both sides of my family have Confederate connections; in fact, George Dixon is part of my father’s line. That said, I am firmly a Lincoln man through and through.
Thanks to their being so much literature about the US Civil War, it’s possible to feel like I knew some of the personalities involved. Plus they had the best nicknames: Hancock “the Superb,” Joe Johnston “the Gamecock,” “Little Mac” McClellan, “Prince John” Magruder, “Beast” Butler, “Old Slow Trot” Thomas, and so on. Plus there is the absolute most Southern name imaginable: General Pierre Gustave Toutant Beauregard.
WW2 just for the magnitude and scope of the conflict. But I’m no history buff.
Also its nice to have a war where the enemy really were bad guys. (not that we were perfect by any means)
Civil War. Take all the crap the Founders didn’t want to deal with during the post-Revolution Constitutional Convention, wad it up into a ball and punt…
…it all lands 70 years later, and unfolds into a gigantic mess that nearly tears the country in half (actually did, for a bit; it only took killing more people than all our other wars combined to bring it back together), reshapes Federal/State balance-of-power, and leaves a historical/cultural legacy we’re still arguing about and fighting over to this day.
I chose “Other” - better known as King Philip’s War. Years of extraordinarily vicious fighting, and still (percentage wise) the deadliest war in US history. And it started about four miles from my house. I can stand in my front yard and see one of the battle sites. It’s about as local as history gets.
The Revolutionary War is a close second for similar reasons.
I do have a soft spot for WW2 history, but that is mostly due to my two grandfathers who related their own stories to me.
Although it involved little direct combat (depending on course on how much you want to separate or encompass The Korean and Vietnam wars) the Cold War has always been my particular area of interest.
I also have a thing for fictional treatments of a NATO/Warsaw Pact hot war, and I’m currently reading The Red/Black/Blue Effect trilogy by Harvey Black, though I’m not sure how much faith to put in the realism aspect when the author has the Defcon system counting up instead of down and a Su-25 Frogfoot lighting its afterburners…
I’ve read that the Civil War was actually the first “modern” war, in that it was the first industrial war, and the first truly large scale war fought using modern logistics and communications in the railroad and telegraph, and where the combatants were using relatively long-ranged rifled weaponry, forcing tactics away from the essentially 18th century musket tactics used until then, and introducing things like trenches, repeating weapons, machine guns, etc…
FWIW I said Civil War and WWII mostly because the Civil War fascinates me because it’s so… personal and immediate, and the effects are still with us. I said WWII because it’s so broad in scope; I can read about the air war, the war in the Pacific jungles, the Italian hills, the French countryside, the Alsatian mountains, etc… There were American fighting men just about everywhere the war was fought.
The Great War. Such a horrific bloodbath, such a huge impact on the world.
My father fought in Germany during WWII. Enough said.
I chose WWII because my dad served as a marine in the Pacific, as someone else posted, enough said. If I had two votes, I would have also chosen the Revolutionary War simply because it fascinates me.
Revolutionary. Lived almost my entire life in the Northeast and plenty of exposure to battle sites. From the kitchen window of my parents house on a clear day you could see Valley Forge park across the Great Valley. I know exactly where theWhite Horse Tavern was and the Battle of the Clouds (almost) took place. In New York I lived just a couple of miles from the Hudson River, not far south of West Point, and adjacent to the town of Yorktown where the British were kept from crossing the Pines Bridge over the Croton River. And now here in Rhode Island the Revolution began with theGaspee Affair, before the Boston Tea Party took place. These are just a few of the Revolutionary War sites I’ve been to, walking the very ground that the Revolutionary Army had trod on centuries before.