I am not much of an Anime fan, never watched an Anime from start to finish untill AOT. However AOT is incredible and one of the best shows I've seen in years, a lot of it is down to the incredible character development.
I think Reiner is the best example of that, his character arc is truly incredible, the inner turmoil he's experienced, the flashback scenes which revealed so much about him, his entire demeanor of a defeated man throughout season 4, everything just makes perfect sense. I don't know if I am the only one but I found myself rooting for Reiner even when it was revealed that he was the Armored Titan, I knew there must have been more to his character and there was.
On an unpopular note, I feel that Levi and Mikasa were not fantastically developed characters in comparison to Reiner, Zeke, Jean, Eren and a few others. I feel there were very two dimensional characters, great characters but in a show full of incredible characters I feel they were a bit lacking when It comes to depth.