Regardless of whether you love the term or hate it, the first half of the word "Metroidvania" is well-earned. 1994's Super Metroid for the SNES is so well-crafted, it deserves to be recognized as the mother of a sub-genre that continues to thrive.
Games writer Mark Brown performs in-depth teardowns of games' mechanics and level design, and his recently-published analysis of Super Metroid's level design is a doozy. Whereas I can tell you "Super Metroid's level design is great," Brown goes into great detail about the choices Nintendo made while making the game, and why they were made.
One point Brown examines and I never considered is how Nintendo holds your hand quite tightly at the start of Super Metroid, then gradually releases until you're allowed to finally run free. You're caged into one corridor until you get the Morph Ball and the Bombs. Then you're allowed to see more of what Super Metroid has to offer you by opening new areas with your acquired powers—but you can't get there until you sufficiently expand your toolset. Finally, Nintendo takes off your leash when you acquire the Power Bombs. Not only do Power Bombs open orange doors, but they also rip open vulnerable walls and let you easily access secret areas you might've missed the first time around. You're free to head into any direction, and the game even celebrates your newfound powers by changing the music on Zebes' surface from an ominous drone to Samus' own heroic battle theme.
Watch Brown's Super Metroid analysis, then take in more of his videos about level design over on his YouTube channel.