Cthulhu.
Both need MLB in order to reach 20% def down of their respective frame (15% without), but Ryuuou's A frame is easy to come by when you join a pub raid. Cthulhu's C frame OTOH, is a lil rarer. Not to mention if you're really desperate for a the extra 5% A frame def down, you can always just slap on Ambush.