One Be Lo for sure, Lloyd Banks was part of 1 of the biggest mixtape runs in the early 2000's, GZA was part of Wu-Tang & dropped a album that is mentioned as 1 of the greatest LP's ever (Liquid Swordz).
Trag is under rated but does get a lot of credit for creating a QB sound & influencing rappers like Nas, Mobb, Mega & N.O.R.E etc.
One Be Lo's Master of Universe (Binary Star) & S.O.N.O.G.R.A.M were very pinnacle for the underground scene, which often gets over looked & not discussed.
If i had to rate them in order of being under rated it'll go like this...
1. One Be Lo
2. Trag
3. GZA/LLoyd Banks.