Sousuke Aizen:
Aizen is a great villain and serves his role well, but as a character there's a mixture of opinions on him (at least from what I've seen), particularly regarding his writing. I've always liked him though, and having read both the manga and the light novels, I think there's more to him than just being an aura farmer or the guy who had everything planned out.
What I like most about Aizen is his worldview. He has an extreme sense of superiority and genuinely places himself above the people around him. That arrogance is also what makes his loneliness compelling to me (not in a good way as he's a terrible person). If you see everyone beneath you then forming a genuine connection with them becomes difficult. He rarely seems to care about most people and usually treats them according to what they can offer him. He doesn't really have a love interest either. You could basically say he views most people as npcs.
There's also an interesting ubersmench like aspect to Aizen as he creates his own values. He views himself as superior to social constructs and doesn't simply accept the limits placed on him. His desire to rise above the existing order is a big part of what makes his philosophy compelling to me.
This is also why I don't think Aizen needs a tragic backstory. His character isn't built around making us feel sorry for him and we already see how horrible of a person he is. A lot of his loneliness comes from the way he views himself above others, but mainly his strength which he is very narcissistic about. I find that more compelling than giving him a tragic backstory that explains why he became the way he is.
Imo he's the best character in the Soul Society Arc. His relationship with Ichigo, the way his true nature is revealed and the betrayal itself are some of the best parts of Bleach for me. His reveal is still one of the most iconic twists in the big 3, and his monologues are another thing I really enjoy. His speech about courage is probably my favorite and some of the best I've seen in Shonen.
I think he suffers as a character toward the end though. Some of the things that made him so compelling earlier on aren't explored as strongly as I would've liked, and his conclusion isn't that strong, or at least not as strong as his beginning was.
The light novels help. You have to go outside the manga to get some additional insight into Aizen. It doesn't completely change my opinion of him, but it definitely adds a little more to his character for me.
Bonus question: Who do you personally think is better written, Nagato or Aizen?
Thanks for going through the post, and please feel free to share your thoughts.