You get the chance to become the Ultimate Icon after you have become the Icon for everything else by getting 100% on all 5 story arcs. (Outlaw, Speed, Style, Build and Crew).
Upon 100% completing all 5 story arcs you will get a call from "Robyn (Crew)" who invites you to a 2 lap, 15 mile race against your entire crew, Manu, Spike, Amy, Robyn and Travis. You must finish top 3 in this race!
Upon completing this race, Robyn will call you again but this time with a 20 mile long sprint race in mind. This is against your crew again, plus all of the Icons you have "dethroned". You have to win this 11 participant race to continue.
After completing the sprint, Travis will call you, and invite's you to the club (garage). Upon arriving the final cutscene will play, and this achievement will pop while the final credits are rolling.
Edit: Spilner commented saying if it doesn't pop during the credits sequence you can skip and it will pop while in the garage afterwards.