I'll start with the only lyrical fansong that I actually like,
The One Who Made Him Die. Where most (like Undertale The Musical) like to commit the cardinal sin of just taking an instrumental song from the game and then singing vocals on top to the pre-existing melody (that's not how it works. if songs on the radio had the singers try to perfectly mimic the melody of the instruments it would sound like shit), this person seems to know what they're actually doing. It uses an original arrangement of Lost Girl as a base, and the lyrics sound like cryptic lines of a poem, and it really captures the whole ominous feel of Snowgrave they were going for. They also made a
second version a year later, which trades a bit of the atmosphere of the original in exchange for being more melodic and adding a new segment near the beginning. It sounds pretty good, though I think I prefer the first one since it's more unique.
From the same person,
Boltzmann Brain is 7-minute-long orchestral Gaster theme that slowly develops and changes throughout its runtime. The weird way it's structured and the various tracks it incorproates and how it changes up a little as it progresses represents Gaster pretty well, and I like the way they arranged the CORE motif.
Speaking of Gaster themes,
Null and Void is one of my favorites. It's from Undertale: Halloween Hack, an AU that exists as a few pages of a webcomic, a bunch of scattered lore and story tidbits from the creator's socials, and the only complete parts being the amazing soundtrack, which seems to be how it goes for these things. UTHH in general has a
really good soundtrack that you should check out, and chances are you've already at
least heard No More Nuzzles.
PARAMALGAM is technically more Pokémon than Undertale since it's an Amalgam-style arrangement of the Paradox Pokémon theme from the latest Pokémon game, but it comes full circle anyways because that was still a Toby Fox track.
Vs. Susie is one of the best tracks in the series, and I don't think I've seen a single cover properly capture the raw emotion from the guitar in the chorus(?), so instead
here's a remix I liked that went in a different direction and played around with the instrumentation instead.
SPAMTON CD is a Sonic CD-style remix of some of Spamton's themes, combined with a couple classic Sonic tracks. Meanwhile,
this is a cross between World Revolving and Sonic Rush's main theme that sounds really good.
ASGORE sounds really good on accordion, and this
Spear of Justice cover uses a really unique selection of instruments compared to most.
And for a couple of stupider covers, I can't help but love these
Otamatone ones, Smooth McGroove's
acapella covers are better thsn they have any right to be, and hearing an epic final boss theme on
kazoo never gets old. Hyuman's
traditional Japanese-style covers are also very nice, and I can't make this long a post without throwing the
DK Rap mashups.
Thai McGrath makes anime-style openings for various popular franchises using music from the series. He has some real bangers, but too many of his tend to devolve into what's best described as "leitmotif soup" as he tries throwing way too many ingredients into the dish. His "TV-size" ones are less prone to this thsn the full-length versions, and thankfully his
Genocide (especially that bridge in the middle) and
Deltarune themes are pretty good.
This remix of
THE HOLY really enhances the ethereal vibe it already had, and this
cross between World Revolving and Field of Hopes and Dreams is pretty good.
I have no idea what the fuck a "swapspin" or
"Teralazing" are but this one is pretty catchy.
Here are a bunch of siivagunner/siiva-style rips I liked, in no particular order. The first one is unironically good, the rest are either shitposty or have the asterisk of being associated with something else.
https://youtu.be/-7gGrnBIP40
https://youtu.be/RyPJ3WaKolc
https://youtu.be/neYjH4BzCtA
https://youtu.be/lRVmUnSRgP0
And finally, though it's technically official, the orchestra
5th Anniversary Concert would sometimes seamlessly blend related tracks together if they were next to each other in the OST, and deserves a special mention if you hadn't listened to it already.