Donkey Kong 64

The Donkey Kong franchise hadn't seen an honest-to-god platformer in three years. The titular character hadn't even been the star of his own game in almost twice that span of time. With the Midas touch that Rare seemed to possess with the genre, combined with being the codifiers of the collectable-centric game thanks to Banjo-Kazooie, it wasn't ever really questioned if Donkey Kong 64 was going to be good- the question was just how good it would be.
 
Donkey Kong 64 is, to its credit, a technically proficient game- taking advantage of the Expansion Pak to render some of the best graphics on the system. The audio is top-notch, the controls are tight, and the levels usually put a unique spin on concepts that were being milked for all they were worth for video game levels. 
 
But the albatross around Donkey Kong 64's neck, and the reason this game misses out on a better rank, is the very foundation that the game is based on- collecting everything in sight. Up to this point, games had kept things relatively simple. Mario 64 had coins, extra lives, special caps, keys, and Stars- most of which were self-explanatory. The basic codifier of the collectathon formula, Banjo-Kazooie, had Jiggies, Jinjos, musical notes, red and gold feathers, and eggs, along with various once-off items used to collect more Jiggies and musical notes.  These games had one item required to progress through the game, and the rest were there to help get more of that one item.
 
Donkey Kong 64? You collect Golden Bananas to be able to unlock the next world- 5 in each world, 5 per Kong, leaving 25 Golden Bananas per world. But to even get to the lobby of the next world, you have to collect the Boss Key by defeating the bosses of all the unlocked worlds up to that point. To even get to the boss, you have to collect a certain number of bananas color-coded to each Kong- 100 per Kong, 500 per world. To be able to beat the final level without running out of time, you have to collect blueprints from unique enemies across the level- one per Kong per world. To get to the final boss fight, you have to collect four Crowns from Battle Arenas scattered across the game, as well as the secret Nintendo and Rare coins for beating the retro games- meaning you have to collect at least 15 Banana Medals by collecting a certain number of bananas per world.
 
And that's not even counting the coins to buy new moves with, ammo for your guns, music power for your instruments, Crystal Coconuts for special moves, Banana Film to capture Banana Fairies, Banana Fairies themselves.... the game holds the world record for highest number of collectables in any game ever for a reason. 
 
The game doesn't require you to get a 201% completion in order to actually beat the game- you only need 100 Golden Bananas to access the final world- but the sheer dearth of collectables in such a high-profile game made this specific game the turning point for collectathons. The subgenre began to decline, and while it didn't die it would still have some life left in it... at least until Star Fox Adventures put almost the entire subgenre into a coma for the next generation of consoles.