Most villains take a whole freaking lot to kill. A sacrifice, a twist, an all-consuming plot or a bad guy suddenly turning good are usually good weapons to kill a badguy with. But these guys? These guys are the ones that make you shout "Oh COME ON! That's all?"
Rasputin (Anastasia): The guy was killed twice. The first time was the lamest: he couldn’t even hold onto a little girl, and then he falls through ice and just…dies. Cold water kills him. The second time, Same Little Girl (Grown Up) smashes his Lava Lamp of Power and he’s dead for surezies. Snooooore.
Bane (The Dark Knight Rises): This villain wasn’t the most fantastic of the Nolanverse’s Batman foils, but when real-third-movie-villain Talia al Ghul came on the scene the film dropped Bane like he was hot. Catwoman just shot him. He didn’t even have any cool last words. No matter what you thought of Bane as a villain, that’s just harsh.
Boba Fett (Star Wars): As kids, we all saw Boba Fett in Episodes IV and V and we thought “That’s an awesome bad guy.” He’s who we think of immediately when anyone says "bounty hunter." But when he actually uses that gun and jetpack and wrappy rope thingy, he gets beat by Blind Han Solo. Not even normal Han Solo. Blind. Han. Solo. (Although, according to the comics, he didn’t even die then. Bummer.)
Venom (Spider-Man 3): Pipes, a pumpkin bomb, and friendship. All it took to kill this guy. Oh yeah, and the effects were terrible as poop.
Saruman (The Lord of the Rings): Saruman was cool and smart. He had a fort. He made super orcs. He brought the Industrial Era to Middle-Earth. But then he sent his whole army out and while they were away, he got beaten by trees and Gandalf came and snapped his staff in half. He's then reduced to a hate-filled, half-mad crazy guy who tries to get some revenge on the Hobbits by (in the books) taking over the Shire—a story arc that makes you almost feel bad for him so long after Sauron's beaten—and he ends up getting stabbed by a guy whose last name is Wormtongue.
Did we miss any?