What do you think was the best decade for these genres?

Horror...1980s by a pretty good margin, giving us fresh and less predictable stories with Aliens, Nightflyers, From Beyond, and the classic Desyat Negrityat, cleaning up after the disastrous hate mongering Hollywood formula horror movies of sixties and seventies that were mostly just neo Nazi Hitler worship movies.

Action..is tough. 1950s is the decade that didn't bore us as much with motor vehicles as other decades. The crack heads of the sixties and seventies used to brag that car chase scenes replaced horse scenes. Yes, they were that stupid. And "no", no car chase scene could ever be as exciting as horses to a sane person. We got some action in any decade, so the "horses over autos" of the fifties gives it the clear edge.

Sci-fi...1960s by a close call over the 1950s. A bit of self effacing and humility helped the sixties get that edge. The fifties gave us the absolute best in THEM! and the third best in UNKNOWN WORLD, sixth place all time sci fi in WAR OF THE WORLDS, but the humility of classics like 12 to the Moon, First Spaceship on Venus, Island of Terror, Island of the Burning Doomed, Robinson Crusoe on Mars, and many others gives a bulk of 9/10 and 8/10 sci films that has never been duplicated

Western...1950s. There is absolutely no doubt about this, and anyone who claims different is either named for Southern trash like "Stonewall", or else a low down Yankee liar.

Comedy...1930s by a slim margin over the forties. Thirties had the Marx Brothers, kings of comedy, at their peak just before 1940, with At the Circus, Room Service, Opera, Races, Horsefeathers. The forties had some good stuff with Hope and Kaye, but comedy began losing its punch decade by decade, with weak comedy in the sixties, a bit of a comeback with Saddles, Rocky Horror, Holy Grail, and a few others in the seventies, but after that, the eighties were worse than weak, and almost nothing funny has been made since then. You can count comedy movies since 1990 with more than three laughs on the fingers of one hand. When you get more laughs out of action than out of comedy, you have to admit comedy is dead.

animation...not my area of expertise, so I'll go with the 1980s for Roger Rabbit. I can be persuaded otherwise, though.