This is a hard one to decide, because I love me a rebel, but in the end it's about favourites. And while the Vorkosigan saga is "Miles!" first in most people's heads, I adore his mother, Cordelia, and this first book is primarily about her.
I think one the reasons she's such a memorable rebel is that she starts out totally buying the party line of her own government, but eventually the indoctrination can't stand against the evidence of her own experience. Even then, most people (and many characters) don't have the strength of will to openly rebel as she did. She was a well ranked military officer, with a comfortable life and family, and she walked away from it all. It's much easier to just close one's eyes to injustice and pretend you don't know, to carry on with a comfortable life.
A lot of people characterize this as all about the romance, for the simple fact that when she walked away, it was to a man. But it wasn't for a man - she'd had that offer on the table almost since their first meeting, and if that's all there was to it, she'd have done it already. So for me, Cordelia is one of the strongest characters in the entire saga, first able to give up the man for what she believes is right. Then when faced with the truth, willing to admit that her beliefs, indeed her entire belief system, was wrong, and to walk away from it.