"underrated" has always been an extremely nebulous term for me so I'm assuming in this case it means "somewhat unknown".
I guess technically the most underrated album I know of has to be Tomorrow does not exist by I broke my robot. But while it has its moments and it does sound incredibly consistently clean and polished I wouldn't call it my favourite, although I do like how disturbingly robotic it sounds. I don't know it's just kinda eh for me but it's nice to listen to sometimes.
I don't know if as underrated as the previous but P H ▽ T I S M S by Tusk Mite would def be my favourite sort of "unknown" album right now. I would say it's very likely the best modern breakcore album out there. Every track feels like a constant climax that just keeps relentlessly building up intensity (with a couple breaks here and there just to let you process what just happened of course). It's kinda like xanopticon but with outstandingly pretty soundscapes and hypnotically beautiful melodies. Honestly it almost feels less like an album and more like an hour long track divided into various different sections. Autumn Continuum, Too Close and Avalon are probs my absolute favourite tracks from it. (I didn't mean to write such a long paragraph here so that probably means I should stop before I write an entire book on this lol.) It is a very nice experience.
anyway, honourable mention to gridlock's formless, it is a very pretty ambient album and chrometaphor hits hard, also literally any album by kid606 but specifically down with the scene.