There's quite a few I heartily enjoyed over the years, so it's tough to pick just one. This is just one of many: CrossCode
I do get a kick out of playing fake (read single-player) MMOs, and CrossCode definitely is a standout title in that regard. While the overall world setting and story might not win any awards for originality, they're still very well done. Yet it is the big cast of likeable characters and their (often) genuinely funny interactions that make it a very fun romp.
Coupling that with superbly done pixel art, great sound design, tight and satisfying action-RPG gameplay, and a slew of well-designed environmental puzzles, this makes for an excellent package. Overall, I'd say it was not just a standout indie title, but one of the best games I've played in the past decade. :) Take that, AAAs!
Lastly, after inital feedback from the gaming community, the developers added much more granular adjustments that can be made to both combat and puzzle gameplay; this makes for much greater accessibility for those that may have a tougher time with some of CrossCode's elements. Frankly, I don't think I've ever seen this high level ability of difficulty-fine-tuning in a game of its kind before.
That said, I highly recommend a gamepad for play. While the game does feature decent keyboard & mouse support, I found the gameplay much tighter and accurate with my XBone pad.