It's really hard to pick one. I love some things about Metal Sonic, I like everything about him, but he doesn't have much screentime to develop. Tails in the Adventure games is incredible in my opinion, I love his growth in these games. A lot of people don't talk about Sonic's character, probably because he doesn't have any flaws like some of the other characters, but I think Sonic, when written well, has a lot of potencial to be an amazing character. In the end he's just a guy who loves adventure and his friends, I like this "absense of depth" in his character, he is the glue that unites everyone because he is just a force of pure good. I also like Chaos a lot, specially his friendship with Tikal.