I'll hazard a guess at Ocarina of Time, with the first clue referencing an unofficial port?

Yup. It's called Ship of Harkinian, it's definitely the best way to play it.

Guessed by @drbunnig

And for the second year running, Ocarina of Time reaches its highest position on this list save for the very first time I participated in the event. That time, this game found its way into the top 20, and while I'm pretty sure it'll never break in there again, it just has this way of staying around. It's been well over a decade since I first played through this game and then replayed it dozens of times. I spent a long time saying it was "overrated" and, well, I suppose back then it kinda was, because the internet seemed really obsessed with the idea that this was the undisputable greatest game ever and if you disagreed you were just wrong. That's no longer the case though - now this is just an adored classic, and all I can say is, it's a classic for a reason.

FULL LIST >>

Ehm all wrong actually but you were right on Undertale being 14 instead so I'll give you that one

Nier: Automata

Current list - https://gamrconnect.vgchartz.com/thread/248646/post-only-once-the-official-thread-the-15th-annual-greatest-games-event/4/#4

30
- Unlike the previous titles in this series, you get to pick the order you complete some of the levels in this globe trotting adventure.
- A remaster was released on the main character’s birthday this year, along with two other games in the series
- The next three games in this series will be getting the same treatment, with their release due at the same time next year.

29 (incorrect guesses - STALKER: Clear Sky
- You get a full time stalker this time round, rather than part time.
- You get to view some familiar places from a different perspective.
- A remake of perhaps the most beloved ‘old school’ game in this series.

27 - Super Mario Galaxy 2
- Unlike the previous game, the unlockable character gets his own unique set of missions in this highly regarded sequel.
- Many fans were frustrated that, unlike the previous game, this was not made available on the developer’s current hardware
- Like its predecessor, this game received huge critical acclaim.

25 - Resident Evil 2
- One particular port of this game is often cited as highly impressive for what it was able to achieve. Many didn’t think it’d be possible to cram all that content on to a cart.
- Once you complete the game with one playable character, you get to play the corresponding scenario with the other playable character.

24 - Resident Evil 3: Nemesis (incorrect guesses - Silent Hill: Origins)
- The events of this game bookend those of the previous game, and also bring an explosive end to this particular chapter in the series’ overall story.
- You are pursued throughout the game by one of the most notable enemies in the series. The name of this enemy is in the game’s title.

23
- One of the titular characters in this game first appeared in another game of a different genre.
- This character can take flight in both this game and the other game mentioned above, although the method in which he does so is different.

22
- The name of the NTSC version of this game is surely a reference to a legendary heavy metal album.

21
- You can see the site of the final conflict draw closer and closer with each world you complete.

27: Super Mario Galaxy 2

25: Resident Evil 2?

24: Resident Evil 3?

Correct on all three

22. Diablo 4. 

Despite some rough changes via patching I think they nailed it. At least the campaign and season 2. Since it's an evolving game I can't speak past that but 450 hours in the base games tells me, I really loved this one.

21. Skyrim

I really don't need to explain why this is on here, it may have aged like milk but I can't seem to fault it for it. 

20. Xcom 2 War of The chosen

Took an amazing strategy game and made it a masterpiece, add in the two other DLC and this game is hundreds of hours of fun, very repayable and nails everything it tries to do and then some. It just doesn't miss the mark, every detail is executed perfectly, it feels like it was linearly crafted no matter how you tackle the campaign. The only thing that would improve this game is a revamped ending mission that's not a so unbearably chaotic. 

18 - Pokemon BW, though XC3 does fit the description as well