Ever loved a certain Pokemon so much? Have you also noticed that whatever Pokemon that is it isn't really loved as much as you love it? Here's the place to discuss that.
To get to the point I'd have to say Aggron is one of the most underrated. Sure, it's got an awesome design and is shown to be very powerful in the anime, so what's the underrated part of it? The fact that so many people overlook it for others because of it's double weaknesses to both Fighting AND Ground. Having double weaknesses to both of the two powerful offensive types in the game isn't something pretty.
Honestly though, the Steel/Rock typing causes more help for me than harm. First you must stare at that 180 Base Defense and double resistance to Normal and Flying, as well as a solid immunity to Poison. It's high Defense also lets it wall Dragon types that don't pack any Ground type moves to do anything about it. In fact, unless it's being hit with Super Effective damage or by a Special move, it doesn't take too much damage at all. It even gets a Special Defense boost in sandstorm to patch up that low Special Defense of his. Sure it isn't the fastest thing around, so that may also be a reason why it's so overlooked and underrated as it is, but you GOT to love that 110 Base Attack with access to Head Smash, which it can deal massive damage with no penalty thanks to Rock Head as one of it's abilities.
Overall Aggron to a lesser extent fits as an underrated Pokemon, but probably not as much as some other Pokemon in the game. He is essentially both overrated and underrated thanks to his abilities, moves, and typing. Perfect qualities of a late game powerhouse indeed.