This is a hard one too! Thor has so many great qualities, so I'll try to narrow it down to the least I can;
His development
Although the difference may seem a little strange(HA) to some, Thor at the end of Ragnarok compared to previous movies blew me away. I can see how much he's grown as a character!
(Can I just add that I am in love with his new look and powers??)
And let's not forget his sense of humour! Some of his scenes are the funniest in the MCU!
And last but definitely not least; his hair! It's gone from chin-length, to shoulder-length, past that; and now short. I love it in all of its forms tho!
Thanks for reading!