Extremely difficult question. Honestly, I don't think I couldn't honestly give a single answer/response without running back into this thread and changing said answer/response. Few minutes later, same. Few hours later, same. Few days later, same. Few weeks later, same. Few months later, same. Few years later, same. Few decades later, same.
Rinse and repeat.
The 90's (most notably early and middle) were just too deep. I wasn't trying to to rain on this thread's parade, it's just more like when it rains it pours with this particular question simply because we're talking about a pool of possible/potential answers/responses that could just about circle the globe.
Hell, I'm not saying this is my answer, just something that popped into my head. Like I said, I'm sure I'd change this and I would but I'm not going to continiously edit or add new posts in here changing things. up. I'll just add this one album to take your bait, 'just want you to understand it's only a nibble, and an incomplete or potentially inaccurate/premature one at that.
Fat Joe's Don Cartagena
Release Date: September 1998
IMO, this is a classic rap album by any measure of standards. It's IMO Fat Joe's best album, and his best overall combination album of both a mainstream and underground vibe. and again, IMO, the outcome is a grand success. Production is among some of the best releases in '98. The guest appearances come correct. Joe spits some of his best verses of all time (even if the late great Christopher Lee Rios was his ghostwriter or not).
There's other reasons why this album instantly came to mind, 'just wanted to scratch the surface to illustrate some of the why behind the selection. When this album was originally released, it received much more negative than positive, by fans and critics alike. Years later it finally began to receive some of the credit it was certainly due, but most fans/critics still held their original assessment close to heart, and since the album is still not considered a legit classic, that's the flagship reason for me claiming it's one of the most underrated rap albums in the '90s.