Kind of feel like there aren't that many underated games this gen. I feel like everything I could think about is at least loved by a good group of people. I mean yeah there's Titanfall 2 and stuff, but people know about that. Plenty of other niche Japanese games that I haven't gotten around to as well, but on a whole I think things are accounted for.
My choices:
End of Eternity 4k-It was underrated last gen and now this pretty great 4k port is this gen. I waited until there was a physcial copy, but I'm replaying it now and it's still a very good game. Janky as fuck, weird as fuck, and maybe a bit too obnoxious with it's mixing of turn based, real time, and strategy jrpg battle system it has, but it's really unlike anything else. Story is still whatever, but game looks really nice and clean in 4k. I never realized how kind of AAA this game is. Great animation, well directed cutscenes, lots of content, surprisingly good sound mix, and well rendered world. It probably wasn't AAA, but they used thier money wisely.
Gravity Rush 2-I don't think this game would win any GOTG awards beyond maybe for it's visuals. Combat is still whatever, but just less obnoxious now because you can throw things easily. Still, it's a very charming game. One of the best looking worlds on the PS4. Also has one of my favorite boss battles of this gen because of the open world scale it happens in.
Valkyria Chronicles 4-I mean they just made the first one again with a far more anime the fucked up story on less of a budget. But man is that simplified strategy rpg game play really good and rewarding. Also the more "talking heads" production values allowed for the main and side characters to be more fleshed out. One of my favorite games this gen.
Ace Combat 7-I think the game did pretty well, but still I don't think it gets credit enough for how much of a good game it is. Just a perfection of the formula.
Quantum Break-I liked Control, but I think I like QB more. Ambitious and I thought pretty good time travel story with really decent characters. It still looks gorgeous and the mix of powers and gunplay kind of plays like a janky western Vanquish. That dosen't sound great, but it is. Because you do feel more skillful playing this game well then say Gears.
CoD Infinite Warfare-It's a CoD game, but the single player here was really good. A surprisingly very human, positive, and non-nihilistic look at the human cost of war. Ok maybe thats a bit too intense, still it sure wasn't the grim dark shit say MW is. The characters are hopeful and have decent archs. The villains.....are shitty. It's still cover and pop CoD play, but it takes you through one of the best realized military sc-fi worlds while you play it.
Gundam Versus-These games are just really fun. I wish this version had an actual thought out story mode. Thats the real problem.