I think Angel has changed, but not as much as some of the others. That's why I say "least changed". Least changed doesn't mean "not changed at all" and it doesn't mean anything bad, either. I love Angel. A lot.
Angel has gone through several changes, and come full circle, as it were, back to the same basically good guy/champ he was before, even a little better. But basically the same. He was a great guy seven years ago and he's a great guy now. He didn't need much character development to become a hero. He was a hero.
Same with Giles. ( And I loves me some Giles, too.) A little wiser, a little more open, a little more free-spirited, a little sadder. But, basically the same great guy he was seven years ago.
Xander. Everyguy then. Everyguy now. Seven years older. He grew up just like his peers, but he didn't change as much as any of them. He changed, yes, but not as much. That's the way he is. That's his role.