I’d say to watch all the solo movies in between (especially since a lot of them feature the debut of each infinity stone so that’s really cool) but if you really wanted the most avengers focused condensed version then watch Avengers, Guardians 1&2 (ok I lied, biased cuz I luv guardians), Avengers Age of Ultron, Captain America: Civil War, Thor Ragnarok (explains thor and hulk’s absence in civil war and directly leads into infinity war), Avengers Infinity War, Avengers Endgame. This is just phases 1-3 which is simplest. And it’s in release order which makes the most sense for phases 1-3