First Avengers film was a very important step in the MCU but I don’t think it holds up particularly well on the rewatch. I’d place it in the good not great category. There are some
really good scenes and it’s great seeing all those remarkably well-cast characters come together but I don’t think it’s one of the top tier movies in that MCU run.
Guardians, on the other hand, is consistently entertaining and one of the best MCU films. I think that once you get past Iron Man 2, you have a lot of the best MCU films in relatively quick succession- Winter Soldier, Guardians, Ragnarok, Civil War, Infinity War. For me, Doctor Strange, also in that run, is quite good. Sort of underrated. I find it to be one of the better origin story MCU films.
Basically, I not only think Guardians is better than 2012 Avengers, I think the 2014-2018 run is probably peak MCU.