Alf, Sally, Don, Pippa, Ailsa, Irene, Morag, Marilyn, Brax, Leah/John
Brax is noticeably shorter (approx 5 years in total) yet he's definitely the most iconic new character of the last 15 years. Him -as well as the rest of the Braxtons really defined that era of the show and even to this day in terms of more regular crime and gangs.
I couldn't decide between Leah and John. Leah has been here since 2000 so really feels like the start of the new millennium. She's been through multiple eras. John does feel like this century's Donald Fisher to me. Both started as these nasty villians (and initial guest characters) that had a lot of power within the roles in the community, that eventually mellowed out and being foster parents. Both had neglected their son/stepson which leads to years of resentment and hatred.