1st Avenger is def the first movie. Introduces the Tesseract right away, Howard Stark, Vibranium, Hydra, Odin etc. And really makes it all come full circle w/ Red Skull in IW
The Incredible Hulk is canon. Has the super soldier serum, Stark Industries and has Ross and Stark cameo at the end. IM 2 and Thor are naturally next as Coulson mentions he has to get to Mexico.
After Civil War, is where it gets grey. BP takes place a week after. Spiderman naturally after that.
I put Wasp next since the majority of its events take place 2 years after CW. Just dont watch the end credits scene. I put Dr. Strange after that to give it more continuity as he is in Ragnork. And Ragnork is naturally the last movie before IW cuz of the Asgardian Refugee ship