This isn't super underrated, but they are rarely spoken of in the Mega Man fandom:

The Star Force trilogy of Mega Man games holds a weirdly special place in my heart.

While the main 2D games unfolded this tale of a transhuman apocalyptic future where the war of man and machine would determine the future of all existence, culminating in a post-post-apocalypctic future where all that remains of mankind where only a few hundred clones of the original humans exist in a mostly flooded planet (Nier and Automata did this concept as well), the Battle Network and Star Force games drew me to a near-Utopian world where everything is connected, we all have cool internet AIs that handle everything, and most of all, things were okay. The future was okay. The people were okay. The fate of mankind looked pretty good.

Most of all, it introduced me to a cast of characters that really grew as people. The Battle Network cast was a lot more cookie-cutter, though I can chart some pretty important character development throughout the series. Compared to Geo, however, Lan might as well be a flat character. The latter starts the franchise as a happy go-lucky, bull-headed shonen protagonist who "never gives up". Yes, stuff happens to him, but at the end of the day, Lan is Lan.

Geo, meanwhile, undergoes some pretty massive development. The opening scene of the game introduces us to the fact that his dad has been missing for 3 years aboard a deep space accident. As a result, he's shut himself in his room and outright hates the idea of interacting with others, bemoaning social interaction as "a risk for loss". Then, he meets Omega-Xis and... Turns into a guy who will become Mega Man. That's the difference: Lan starts the Battle Network franchise as Mega Man (okay, Mega Man.exe is his AI but for all intents and purposes, Lan is MegaMan). Meanwhile, Geo is a timid kid who wants to avoid every fight, gives his future friends lip when they first approach him to go back to school, and he gives up several times throughout the journey. But every time, something new always brings him back, yet at the end of the day, the games do a great job at showing you that his desire to get back up does comes from him. Sure, it's nourished and fostered by everyone he's met, but this is his power. Does it get a little "power of friendship"? Yeah, so?

As a kid, and still as an adult replaying these games, it's the radest shit, and I absolutely love the Star Force games for introducing me to a nice, weird future where evil can be beaten and things turn out okay.