X2: X-Men United


Professor Charles Xavier and his team of genetically gifted superheroes face a rising tide of anti-mutant sentiment led by Col. William Stryker. Storm, Wolverine and Jean Grey must join their usual nemeses—Magneto and Mystique—to unhinge Stryker's scheme to exterminate all mutants.
Extremely iconic sequel that surpasses the first. If you mention X-Men this is the first thing that comes to mind. Good story development.