Heltah Skeltah is my favorite. Hard to choose with those heavy hitters, but I listen to Nocturnal more than all of the others. Also Sean Prices solo albums are great.
Mobb Deep got underrated because at the time NAS came out with a similar NYC style, and he got all the shine. Understand NAS was the streets, he was hip hop. If they came out before NAS more people outside the east coast would have noticed.