Theoretically, all champions can use this rune, but not everyone makes good utilization out of it. Champions that depend mostly on basic attack damage do not blend well with Arcane Comet. This is because the rune procs while hitting your enemies with abilities, not with basic attacks.  Most of the champions that rely on their abilities are mages, making them perfect for Arcane comet. Also, champions that are not ranged mages, but rely on abilities, can make good usage of Arcane Comet. For example, ADCs like Miss Fortune, Ashe, and Varus possess the qualifications to make this rune look good in their category of champions.