Guessed by @UnderwaterFunktown
This should have been the greatest Metroid game.
It absolutely perfected the foreboding atmosphere the franchise is known for. It managed to bring in alien characters and dialogue, making its story a lot more engaging than its predecessor, without losing anything in terms of lore or that feeling of isolation that defines the series. The art direction is breathtaking and evocative, every room, every area not only looks beautiful but gets you thinking of what used to be. Unlike the somewhat disjointed world of the first game, Aether feels tangible and connected, it feels like a real place, and you can see how the Luminoth used to live there. And the smaller-scale level design is fantastically realized too, with an insane amount of twisting interconnected rooms and corridors that are impossible not to get lost in - it's overwhelming, but so very Metroid.
Unfortunately, for as much as it gets right, Echoes never reached its fullest potential because of a few bad decisions and a rushed development cycle (not helped by the attempt to force in a multiplayer mode, I'm sure). It is clear to see where corners have been cut, especially when you see the blocked passageways in the dark world, the type of thing you'd expect to find an upgrade for, but it never comes. Speaking of the dark world, it is single-handedly the worst thing about the game - never mind having the gameplay interrupted by loading screens, the main issue was the decision to make the player heal (very slowly) inside the light beacons. I get that they'd risk making the game way too hard without that, but what they've done instead is replace the dark world's tension, with boredom. Because let's face it, we all just sit there inside those beacons for minutes at a time to heal. And limited ammunition on the special beams sucked.
Anyways, it's hard not to feel disappointed by Prime 2, because it is so good when it's at its best. And to be fair, even with its issues, it would've still been the best game in nearly any other series. But Metroid games are very high quality, and they don't often do "flawed". Echoes does, and that sucks, but this is still an amazing game, and outside of the few things it gets (quite) wrong, it honestly should be seen as the blueprint for what a Metroid Prime game should be.