Some people watch the Star Wars movies and are impressed with Han Solo's coolness. Others are struck by the heroism of Luke's journey from farm boy to Jedi knight. Still others can't think of anything other than Princess Leia in a metal bikini. (And that is pretty nice...) But for my money, the biggest hero in the Star Wars saga is R2-D2. Lemmie tell you why.
When you look past the lightsabers and laser blasts, it's clear that the entire story hinges on R2. He is constantly pulling the good guys' fat outta the fire. From the moment he pops up in Phantom Menace, saving Qui-Gon and Obi-Wan's ship from being blown to bits to zipping Luke's lightsaber to him moments before he's pushed into the Sarlaac pit, R2 is constantly making the difference that saves everyone's asses. He keeps Padme from being incinerated in the droid factory. He throws down with a pair of super battle droids and hands their asses to them.
It's not just the ass kicking that puts R2 over the top. He's constantly listening to Luke complain about being stuck on a muddy planet. He's an out-of-the-box thinker, as he proved when he fixed the Millenium Falcon's hyperdrive. He's also the only character in the whole saga who actually talks in believable dialogue. It's just that they have to bleep it all out. The pottymouth.
Plus, for all the powers the Jedi possess, none of them can fly. R2 can. And he always has the right tool for the job. He's like a trash can-shaped boy scout who's always prepared for everything. On top of that, you can use him as a movie projector if you need one. And he's indestructible. You can blast him with a blaster, blow him to bits with a TIE Fighter, but give him a few new parts and shine him up, and he's as good as new.
You can argue about the story being about the Skywalkers, or that Han Solo is the bad ass of all bad asses. You can talk about the mythic elements of Darth Vader's story. But there's no way you can argue or no amount of money you can offer that would convince me that R2 is the real hero in the Star Wars movies. Without him, the those other "heroes" would be dead.