Agreed. Ultron should definitely be the villain in Avengers 2 (and hopefully he'll be set up in an Ant-Man movie beforehand). It'd be great if Ultron were to try to pull a Skynet and try to create an army of AI's (which'll include The Vision) to take over the world. Kang would also be a great villain to use for Avengers 3 if Thanos is out of the question for whatever reason. He makes for a nice escalation from the threat which Ultron represents.
Well, if we're measuring the size of Thanos's penis versus that of Korvac's penis, then I accept the challenge, because Thanos has a MUCH bigger penis!
In The Infinity Gauntlet Thanos had already killed the majority of the Avengers, the Fantastic Four, the X-Men, and numerous other heroes before they even knew what was going on, along with half the universe. When they turned up to attack him, he froze them all and was about to instantly kill them when Mephisto talked him out of it and convinced him that watching him fight them would be a turn on for Death, so Thanos purposefully reduced his power to 1/6th its level to fight them, and he not only beat them, he killed pretty much all of them. With the Infinity Gauntlet, Thanos's power wasn't almost on par with the Beyonder... he surpassed the Beyonder. He trounced Galactus, the Celestials, the Stranger, the Inbetweener, Lord Order and Master Chaos, Eon, Death, Mephisto, Mistress Love and Sire Hate, and whoever else I'm forgetting about simultaneously
, and then he took on Eternity, the personification of the universe, and won, becoming the personification of the universe himself.
In comparison to Thanos with the Infinity Gauntlet, Korvac's a piker.