[Barrett]

Hoo-boy. I sent a similar query to a lot of my military aviation pals this summer, thinking there might be an article in it. Most folks wanted 109Fs because of the attendant target-rich environment. A Mustang ace who preferred the P-38 said "An armor-plated Zero might be fun."
WW I: two RFC choices are BrisFit and SE-5. Camel's a mean machine but lost about as many pilots in training as in combat. That is NOT good, especially by the standards of those days. I'd prefer to step from my time machine knowing what I do about aerodynamics and flying technique as I do now--a big advantage in survivng.
Fighting other folks who never did me any wrong simply doesn't matter. They're volunteers as much as I am, and I'd just as happily sign up with the Luftstreikrafte (maybe Jeni would be my squadronmate in Jasta Studmuffin!) Anyway, the whole war was a pointless exercise in futility so the challenge is reason enough to play the game. Let's get it on!
WW II: gimme a radial engine every time. Carrier-based Hellcats fershure: easy to fly and lots of opportunity if you're in the right time and place. For Brand X, FW-190, east or west. However, for an individual's maximum influence on the outcome of the war, there's only one choice--Douglas Dauntless. An SBD pilot could do more to effect the course of history in 30 seconds than boards of admirals and generals in 3 years.
My anytime/anyplace dream machine: Northrop F-20 in a mercenary air force fighting anybody, anytime, anyplace, baby. (See "Warriors", Bantam Books, 1990)