They'd have to be able to effectively itemize high amounts of CDR, like Ryze, Heimer, and Zyra.
They'd have to have high base damage or utility to make up for the lack of offensive stats.
They'd have to relatively unlimited by mana or other resources in order to cast very often.
My favorite would be Vlad using 15.89% runes + masteries, Ionian boots (15%), Spirit Visage (10%), Hextech sweeper (10%), DFG (15%), Rabadon's, and WotA for a total of 66% CDR or a cooldown of 1.4 seconds on his Q and 1.5 on his E.
Edit: oh ****, sweeper is still Dominion only. Guess I'd pop a Kindlegem into that spot and get a elixir of brilliance and blue buff for 96% CDR.