Well, if we're honest (And these are just my personal views on the films):
phase 1 had Iron Man (A), Hulk (B), Iron Man 2 (C), Thor (B+), Captain America (A) and Avengers (A+) - Average score is 3.5 (B+)
Phase 2 had Iron Man 3 (A+), Thor The Dark World (D), The Winter Soldier (A+), Guardians of the Galaxy (A+), Age of Ultron (B), Ant-Man (B+) Agents of Shield (B+) - Average score is 3.5 (B+)
Phase 3 had Daredevil (A+), Jessica Jones (A+), Iron Fist (D+), Luke Cage (A), Defenders (B), Punisher (B+), Inhumans (F), Civil War (A+), Doctor Strange (B+), Guardians of the Galaxy 2 (A+), Spider-Man Homecoming (A+), Thor Ragnarok (A), Black Panther (A+), Infinity War (A+), Ant Man and Wasp (B), Captain Marvel (C), Endgame (A+), Far From Home (A) - Average score is 3.5 (B+)
Phase 4 has (so far) Black Widow (B+), Shang-Chi (A), Eternals (C), No Way Home (A+), Multiverse of Madness (B+), Love and Thunder (B), WandaVision (A+), Loki (A), Falcon and Winter Soldier (B), Hawkeye (A), Moon Knight (B+), What If (B), Ms Marvel (A) - Average score is 3.5
This is actually the first time I decided to grade and average everything like that. I was pretty shocked to see that the average scores come out pretty even. We can, of course, disagree about whether things like the Netflix Series and the Inhumans belongs more in phase 2 or 3, but I based it on release dates. Overall, it seems the quality has remained rather consistent. Of course your mileage might vary, but I tend to think I'm also a pretty forgiving reviewer in a lot of ways. I based most of these scores on "How likely am I to revisit this film / show?"