Underpowered: All Melee Fighters except Rogue.
The reason for this is because they have no choice but to go melee unless they want to do constant basic attacks for ranged damage. Going into melee is fine, but they will take a lot of damage, and are extremely weak to things like immobilized condition. I place the Rogue as the only exception simply because they have a very high damage output to over come this detriment.
What you don't realize is that without melee fighters, your ranged attackers become front line characters and are not so hard to hit anymore.
You don't have a good grasp of what overpowered/underpowered means. It's not a ratio Damage output / damage taken. It's overall usefulness. If we have a balanced team of 4, the addition of any one class should only improve the party's power by 25%. If a class bring more or less than 25% increased power to the party, that class is a little more or less powerful than the others.
I think most class are reasonably balanced but if we did extensive test with model partys, where we run the same fight several time by removing one different PC at a time, I think you'd be surprised to discover that the archer ranger is probably not that high on the totem pole of the most crucial team member. After all, removing him only reduces damage output but doesn't affect in the least the ability of the team to hold a line or keep itself fitting fit. A fight where you remove a fighter, paladin, warlord or cleric from the mix is probably going to go more poorly than one where you remove an archer ranger.
In fact, the archer ranger almost exclusively provide damage to the team. Most other classes bring more to the table and are therefore IMO more useful/powerful since they also do damage and their additional contribution outweight the difference in DPR.
The melee ranger actually is much more powerful than the archer IMO
because he takes more hits. Beside comparable DPR, he also brings to the table an improved ability to hold the line for the team, spread the burden around a bit and synergize well with the defender marking ability. In a team of 5, you shouldn't have more than 2 purely ranged characters IMO, and the fact that they take less damage than the other 3 should never be construed as being more powerful! It should be seen as a lesser contribution on that front that needs to be compensated in other areas. And in fact, IMO, the Archer ranger's higher DPR isn't quite that much higher to make it worth it to have a PC hiding in the back and shirking front line duty. Warlock and Wizard brings more to the mix IMO and in a team of 5, I'd rather shield a warlock than a ranger from blows.
Taking hits isn't something to denigrate. It's something that has to be done! And if your PC doesn't do it, he better make up for it in some other way. I don't think the archer ranger brings enough to the team to compensate the loss of a warm body at the front and so I think
he is one of the most underpowered class!