Yeah I was watching the first Avengers again last night and was struck by how much more realistic those two armors looked compared to ones in later movies, I think because they were still only replacing Downey's legs at that point, instead of creating the entire armor in CG.
Although the bulkier and more angular armors back then might have helped with that as well, and been easier to make look real.
Simple explanation is that more and more effects companies are being used on each movie, and all the really good looking armors you remember were most likely done by ILM or Weta, up through the 43 and Hulkbuster in Ultron, then it becomes a real grab bag of VFX houses, even within any one film.
Weta did the whole IM3 final battle, while Scanline, Trixter, and Luma did some of the earlier Mk 42 shots, which looked fine.
Method Studios did the noticeably off Mark 45 in AoU, that was their first time doing an armor.
ILM
did do the CW airport battle, and 46 doesn't look up to their standards imo. They did have to digitally replace basically everything in that scene, so perhaps they couldn't give it the time deserved- everything about CW seems crazy rushed anyway (and ILM didn't do a single other scene featuring the 46 or 47). Method did the silo sequence, they got a bit better there and the darkness helps the shaders not look too computery, but not wild about the animation.
The Mk 50 which looks pretty cartoon in earlier scenes was Framestore (NYC battle) and Cinesite (on the ship with Maw). The 50 looks probably the best in the Titan battle, and that whole thing was done by Weta.
Not all the Endgame VFX breakdowns are out yet, but as far as the 85, Weta did most of the final battle, with ILM handling at least the Captain Marvel sequence. Not sure who did the 2012 Stark tower 85 shots yet (Cinesite did the lobby sequence, Lola and DNEG did Captain v Captain).
Iron Man (ILM)
Iron Man 2 (ILM)
Avengers (Weta)
Avengers (ILM)
Iron Man 3 (Scanline)
Iron Man 3 (Framestore)
Iron Man 3 (Trixter)
Iron Man 3 (Digital Domain)
Iron Man 3 (Weta)
Age of Ultron (ILM)
Age of Ultron (ILM)
Age of Ultron (Method Studios)
Civil War (ILM)
Civil War (Method Studios)
Homecoming (Luma)
Homecoming (Digital Domain)
Infinity War (Framestore)
Infinity War (Cinesite)
Infinity War (Weta)
The 50 really needed to look like this (which is a practical suit with a bit of digital polish), but never got there