Aggron was a crucial member of both my Ruby and Black 2 run, and for a long time was a part of my main team. How cool is Aggron? Let's put it this way, there's a whole meme dedicated to it's awesomeness. You'll just have to look it up yourself.
With access to awesome moves like Head Smash, Heavy Slam and Earthquake, combined with abilities like Rock Head and Sturdy makes Aggron a force to be reckoned with. He does unfortunately have a few critical weaknesses that can leave him vulnerable, but with a proper partner setup this can be overcome. He is a valuable asset to any team.
He acquired a Mega evolution in Generation VI, but I'm surprisingly not a huge fan of it. It looks a little bizarre to me. I'll stick with regular Aggron, thank you.
Design wise Aggron is one of my absolute favorites. Who wouldn't want to use a metal plated bipedal triceratops? Plus, he kinda looks like Mecha Godzilla.