10. Composite Superman
What better place to start than with a villain made up of both of the impending film's heroes? Composite Superman is of course the amalgamation of Batman and Superman (and indeed a number of other heroes, who were rather cruelly ignored in his near-iconic - and iconically stupid - appearance) who was invested with powers in his original iteration thanks to a rogue lightning bolt that turned a disgruntled Superman museum employee into a super-powered villain. Curiously, that version of the villain chose to use his incredibly shape-shifting ability to turn himself into various statues, while publically humiliating Supes and Bats, in order to make himself look like a more credible hero, before attempting to take over the world, naturally. Later iterations of the character were a little less complex, with Composite Superman reimagined as a Frankenstein's monster designed to replicate the Justice League's powers, who ended up a confused abomination who wanted to be good, but ended up being tricked by Batman and Superman into ripping itself in half. Zero Hour killed off Composite Superman, replacing him with the less comical Composite Man, a weaponised Durlan who could use his powers to replicate any member of the Legion of Heroes, and their powers.
Why He'd Suck You can probably see the appeal - the only way to take down a villain made of both Superman and Batman is to bring them together to channel the power of super awesome friendship, but come on: Composite Superman is a joke. Despite his obvious and enviable powers, he chooses not to take advantage of them, coming up with a design for himself that would make a big screen appearance an impossibility.