Influential bands that don't get enough credit:
1) X (post-punk harmony started here, for better or for worse)
2) PIL (especially their first two records)
3) Husker Du (No Husker Du= No Pixies = No Nirvana, plus guitarists all over the world owe Bob Mould a debt, weather they know it or not.)
4) Elliott Smith (although now that he's dead, maybe more people will listen)
5)Sinead O' Connor (I can't count the number of female singers that have made a career of imitating her badly)
6) Thin White Rope (If you're not in Nor Cal you won't know)
7) Chrome (they were busy starting industrial music in the 70s)
8) OV Wright
9) James Carr
10) Joe Higgs (Bob Marley's voice teacher)
11) The Ramones (Yeah, I know they're famous, but they were making less than their roadies while their imitators--I'M TALKIN TO YOU GREEN DAY--were making bank)
12) Louis Armstrong--Hear me out now!-- Not enough people recognize his vast contributions to modern music and see him as a funny, gravel-voiced Uncle Tom. And they're really missing something tremendous.
13) Sonny Sharrock--he was a big influence on MY playing and deserves to be known for more than the "Space Ghost Coast-to-Coast" theme. His record "Ask The Ages" with Pharaoh Sanders and Elvin Jones is beautiful and moving but also challenging. I urge you to buy it if you like freer types of jazz.
Now as far as unappreciated songs and albums go, the first to come to mind is Here, My Dear by Marvin Gaye. When he sings his wife's name the third time ( Anna, Annna, Aaann-aaa!) during Anna's Song, it is the most desperate sound ever caught on record.