Of the required ones, theory of gravity in the Middle Ages, usually, and Radio in Conquests 1.00/Vanilla in the Industrial Ages. Ancient Ages it depends a lot - Polytheism if I'm going Republic right away, otherwise usually Currency or Construction. In Industrial Ages C3C1.22, most often Electronics, unless I'm doing well in techs and decided to go ToE -> Atomic Theory -> Electronics, and build Hoover Dam, ideally with a Great Scientific Leader.
Including optional ones, probably Monarchy in the Ancient Age, and Free Artistry in the medieval (unless going for 20K), and Advanced Flight in the Industrial, although Ironclads is likely, too, if enemy frigates pointlessly bombarding my shores aren't annoying me too much.
In the Modern Era, probably Integrated Defence most often. Maybe Stealth. I never actually need them, so I'd sooner get some nice happiness/health wonders that I don't really need with Genetics. Robotics I like because of the increased production, as well as SS part.
I got to like Future Tech 7 in a Warlord Iroquois Vanilla game once. Then I won by Spaceship. Never really bothered with Future Techs after that as I was disappointed with the very low score bonus.