Mass Effect 2 for sure. It should have been a faithful sequel to ME1 and stuck with the awesome tone and scale of the Reaper storyline without the final mission weirdness and roster shakeup. It should have fixed inventory management but preserved the RPGish aspects with weapon choice, armor variety, and character progression.
I also want to name Star Wars Battlefront in its modern form. Imagine if its gameplay was actually good. Like, Destiny or Dragon's Dogma good. Imagine if it had robust content for every cool Star Wars setting. How do you mess that up, seriously? Just throw money at competent people.
I know it can't be that simple, but talented devs exist in the game industry, and Star Wars is a money printer, so...
Dear god, why did no one name Final Fantasy XV? I just looked up "wasted potential" and the game is listed for definition.
Royal Edition makes it pretty decent. The last half of the game is still missing, and the story still steps on the gas like crazy right after you travel to the second major area, but the free roam portion with its setting, characters, and gameplay could be far worse. Royal Edition really fills out the good part and improves the ending.