I don’t really have a great sense of if something is underrated or not, but here are some great series that I don’t see mentioned a lot here:

• Sweat and Soap - This might not qualify as underrated, but it is a fantastic love story between adults
• Love After World Domination - This one was cancelled after the anime didn’t drive sales, but I absolutely loved this series and would revive it if I could
• Love’s In Sight - A fantastic story about two outcasts from normal society falling for each other.
• A Story About a Droid - This one was dropped by the author (it was a Twitter series), but I really enjoyed what exists.
• Ubunchu - An old series that was published in the Ubuntu magazine in Japan. I find it hilarious, but you probably need to fit my exact demographic profile and linux experience for it to hit as hard.
• Slime 300 - I am not fully up to date on this series, but it is a great, slow-life isekai. There are never any stakes and is just a comfy time. Season 2 of the anime is coming this fall iirc.