I read this thread and could not decide my favorite, so I went to my MM shrine, and placed my new sets (dc8 and SOTL) in the cases. Upon reviewing every minimate I own (I have all except gold and silver spidey) I decided that I could not decide. I am not a comic book buff, so I will say the Dark Angel, and here are three reasons: 1) It is Jessica Alba, and I have a slight obsession with her hotness, 2) it was the first minimate ever, and 3) I was most excited about finding this during my "search and conquer" phase (which I miss).
I am not sure what is the most valuable mate I own, but I value the custom flip flop paint mate I made with my brother the most, and it was the first and best custom mate I ever did.
WW4M