Some of the responses here surprise me, such as Shallan & Pattern, as they are main characters in the current storyline. Even suggestions like Rock & Teft caught me off guard as they have such significant screen time. It's true they aren't often mentioned on forum, but that's likely because they are supportive characters without many individual branches of their own. All 4 great characters, just not what I'd consider underrated...
My Vote: Lin Davar
and here's why... (minor spoilers within, reader beware)
A very important detail within my opinion of him, please remember that before he broke, he was a good man, a good father and a good house lord. Also remember that the act that broke him was an incredibly selfless and noble act. Also remember that that act took place around 10 years before the current timeline, about 9 years before his death.
His house was ancient but unimportant (according to Shallan), yet he was making considerable political advancement. He was somewhat successfully positioning himself to make a run at becoming High Prince. He made quite successful advancement within a powerful organization (GhostBloods), enough for them to trust him with a priceless fabrial (SoulCaster). Keep in mind, chances are, both of these were well underway long before he broke himself (Shallan mentioned every time a quarry ran dry, he immediately went out and found a new one, suggesting he has had the SoulCaster for quite some time).
This part is laced with more speculation, but that's often the fun part. We know for certain at a very young age (7 or so) his daughter bonded a spren. Bonded well enough to use him as a shardblade. And not just any spren but the first spren in over two millennia of one of the two most powerful political spren factions. What possibly could have brought this child to the attention of the spren BEFORE she killed her mother?... There is a reasonable theory that Shallan isn't the only SurgeBinder in the family, but her eldest brother Helaran is Skybreaker. Keep in mind that, being the eldest, Helaran had the most association with Lin before he broke himself (and as the gem of the Davar House, Shallan had the closest). Do you find it odd that his two closest children became SurgeBinders? As we've seen among the Kholins, spren like to keep it in the family. Now consider that the guy had a "broken SoulCaster". Where is the only other non-functioning SoulCaster referenced? Oh yea, Jasnah! But it's not broken, it's a fake to cover up that she herself can SoulCast... So is it possible that, maybe, Lin Davar had bonded a spren and could SoulCast? Maybe that was the secret behind his political rise both in both Jah Keved and the Ghost Bloods? Is it possible that while saving his daughters life, hiding her secrets, being hated by his family, being loathed by peers who used to respect him, etc, etc... he broke his oathes and killed his spren... which in turn absolutely broke him as a human being?
I know, I know... lots of speculation in there. Lots of unfounded and unfoundable assumptions. But also no major obstacles in the proposed path. Of course we can't say it's for certain, but possible...
I'm not willing to assume it's true and label him a great man who got totally shafted... but I am willing to say his story worth of great consideration and is thus underrated...