Izaya for me~ Granted Shizuo is built like a gladiator, stronger than an elephant, and about 3-4 inches taller than Izaya. However! Izaya is the maniplative, cunning, and mentally dominant one. Plus Shizuo is a tsundere~
When it comes to strength/anger vs speed/intelligence, the latter always wins for me. Despite Shizuo being stronger and taller, I see Izaya being seme. He seems to always have the upper hand when they encounter each other. Izaya knows how to push Shizuo's buttons, and would probably use his own anger against him. He enjoys messing with Shizuo, and Shizuo's strength and anger would make it all the madami fun for Izaya to dominate.