What episode would it be best to watch before I read the Eugenics Wars books? I know they brought in a ton of stuff from a bunch of different 20th Century episodes, and there's a lot of them I don't remember that well. I already plan on watching Assignment: Eternity and Space Seed.
Those two are the biggies, of course. I managed to reference most of the 20th Century eps, but sometimes just as a cute in-joke or cameo. You might want to check out "11:59," the VOYAGER ep about Janeway's ancestor, since Shannon O'Donnell figures rather prominently in Book Two. Also, "The Neutral Zone," that first-season TNG ep where they find three survivors of the 20th Century in suspended animation.
Also: "Little Green Men," the DS9 episode set during the Roswell incident.