Although I agree with the aforementioned Doc Watson, my top pick would have to the little-known Nick Drake.
His story is pretty tragic, despite growing up amongst England's privileged middle class and attending Cambridge University, he was otherwise dealt a pretty band hand in life as I gather. He was reclusive at school, suffered from depression for most of his life and failed to find a job he enjoyed as much as being a musician.
When he was finally picked up by Island Records in 1969 (iirc) at the hands of folk and blues producer Joe Boyd, all three of his albums performed poorly. Why this happened is (arguably) a mystery.
Take 'River Man' for example, off his debut album:
If you're a guitarist you'll realise why this song it's so good: it's played in 5/4 (which for the uninitiated is an incredibly wacky time signature for most modern songs) with some particularly odd chords whilst being fingerpicked beautifully in English Folk style by the man singing. That takes some skill, plus, it sounds beautiful.
I also believe 'Pink Moon':
With it's dead, soft sounding strings and completely custom tuning by Nick (C-G-C-F-C-E) further proves my opinion.
As does 'Parasite', off the same album, with it's further use of complex fingerpicking and Nick's preferred custom alt. tuning:
Feel free to give your thoughts, sorry for the text wall...
-- Joe