NES Metroid, being replaced by Metroid Zero Mission.
NES Metroid is interesting to play through to see where the franchise came from, or for the nostalgia factor, but Metroid Zero Mission is vastly superior in nearly every conceivable way, its not even close. Its not like Silent Hill 2 or Resident Evil 3, where the originals are still better than the remakes overall, everything taken into account (though in that case, SH2 remake is superior to the RE3 remake). Absolutely every element of Zero Mission is an improvement on the original.
Metroid Zero Mission did not make vast sweeping changes to alter the identity of the game, making only minor adjustments to designs that were not thematically important (for example, the physical appearance of Ridley or Kraid being different is not thematically important). There were not big amounts of cut content, with only minor elements being cut like the fake Kraid enemy, which was not thematically important. The music is all familiar with the same composition, but with added flair. Its not different just for the sake of being different. Items and suit upgrades are almost all in the same places as the original NES Metroid, with the addition of new items that were added to the Metroid setting later on such as the Charge Beam and Super Missile. A map was added to the game, and the beam weapons now stack like in Super Metroid, rather than replacing the last beam you had.
All in all, Zero Mission leaves very little reason for the player to play the original game, especially if all the player cares about is the overall story of the Metroid IP. The player won't get more thematically important designs that enhance the story like they would playing the original Silent Hill 2, and they won't get more original game content and story like they would playing RE3 Nemesis. They wouldn't get an improved experience. The choice to play NES Metroid mostly just comes down to nostalgia, historical value, or personal preference. Or if someone only has an NES or device capable of emulating the NES but not the GBA.