I wonder if you really ask about Michael himself, his character, or if this is the typical question about overall favorite eras. If you mean the second, then it's not really a weird question. The MJ fandom is not the only fandom that has favorite "eras".
My standard answer is normally that Dangerous is my favorite era. And even though it still has a really special place in my heart, I couldn't really decide now. It's probably easier for me to say which eras I don't care as much about. I tend to focus on the J5 era til the Dangerous era at the moment.
If this question really was more about Michael himself:
J5 - Cutie Pie!
Off The Wall - such nice energetic positive vibes. Ray of sunshine!
Thriller - Michael being more reserved, which also makes him seem like the coolest guy ever. King energy
Bad - He gives us "bad boy" fanservice now and dresses even cooler. His mind! Still confirmed cutie pie though. This mixture makes the fans wild lol
Dangerous - feels like the era where he kind of started freeing himself in certain ways. For me it's like this could have been the MJ awakening, I will never stop wondering what could have been, if his career would have continued in a more normal way.
HIStory - Michael finally being tired of bullshit and started being more outspoken. I have to stan!
I also loved his last years (around 2007), he seemed happy again and everything seemed to be possible. "The best is yet to come"