Cato Sicarius was what sold me on Ultramarines. From a practical standpoint, he isn't an overly powerful named IC, but one that's balanced in his own way that matches both the fluff and general game play. The bonuses he gives to a list are average at best, and his wargear and Special Rules aren't that different from the other named IC's in the SM codex, save for Calgar and Lysander who both have Eternal Warrior.
I just enjoyed the exploits that were accounted for by him in the codex a lot, and the opening picture of him and his battle company on pages 18 and 19 were just so enthralling and captivating to me.
Captain Cato Sicarius: Hands down my favorite.