I think all of them changed a lot, but Dale the most. When the show started, it seemed like he lived in a bubble and he was very narrow minded. But then he met new people and experienced new things, which helped him become less judgemental and accept/respect people that were different from him.

hes become less of a jerk.

I think they all changed in some way or another as most characters do as shows go on however i think that Rusty changed the most. He went from being the nerd with no friends in high school, to not only joining a fraternity but ending up it's president, to dating a girl(ashleigh) that he never in a million years would have had the confidence to even be near when the show first started.

It is a sign of how good the show is that all of the characters changed over the seasons but never contradicting their personality, in fact they have all matured, made mistakes, learnt from them, and basically grew up through the years... I think if I had to choose the one for whom the changes are more "noticeable" it's Rusty because he really became more confident in his own skin and he became the person he wanted to be and more than that, the person he liked being.
But all of them grew up and matured a lot... One of the great things about the show is its character development.