Centuria

Nakuru's original songs!
Killer Maid - incredibly catchy, cute but with a side of darkness!
Theater Endroll - a sad and melancholic story, with a lot of layers.
煌命残滓 - "the remains of (a) glittering life". It's like heavy metal but with pretty moments. Also, Nakuru wrote the lyrics herself and they are REALLY good.
JelLaboratory - mysterious, featuring piano and cool harmonies. There is also a story here...
Cosmodiver - fast and rocks hard. I used to drive at night listening to this song, watching the stars just like described in the song.
Bubble Bubble Boy - bubbly, cute, but with a small side of darkness... cute animated MV!
FAKE IDOL - my favorite Nakuru song of all time, and probably my favorite song of all time. Watch the MV, there is quite a story here as well.

Covers
Pina Colada - this is first on the list because it actually comes with English subtitles in the video!! Lovely and pretty song. Keep in mind that she covers this one with her friend Natsume Itsuki, also a singer that I like. She has the brighter (less mature-sounding?) voice.
Ice breaker - same composer as above and pretty much the same subject. She does this one with Itsuki as well. Personally I like this song quite a bit more, though!
Idol - this song topped the charts in Japan for a year after it came out. Definitely worth hearing Nakuru's take on it.
First Love Diary - lovely, pretty, and bittersweet, a sad story about love. She also made the MV by herself! Although I love it, this song also brings back some sad memories for me. I tried to create a parody with this song for an end-of-the-year project last year and it crossed the line to the point where our teacher won't play it... I will sing it properly soon, though! When I do, I will link it here.
Meltdown - catchy and rocks hard. The contents are depressing, but you won't be able to tell unless you can understand. She hits all of the high notes beautifully!
INTERNET OVERDOSE - it's just really good. A popular phenomenon for a while. Watch the MV! (but I wouldn't recommend if you don't like flashing lights)

Endorfin. - a music unit consisting of her on vocals and a producer named sky_delta
蛍火の雪 - "the snow from the firefly's glow"... it's the only recommendation I have that has an MV! The MV is quite pretty.
Glass Ageha - GO LISTEN IT'S REALLY PRETTY. The instrumental is so well-done. I think "ageha" refers to "swallowtail butterfly", but because of Japanese Rs we'll never know if it's actually Grass Ageha or not. I think glass makes more sense though.
Ulysses - this one just keeps getting stuck in my head. It's very catchy.
Luminous Rage - it's kind of like... heavy and rocking-out and catchy guitars? I really love how her vocals sound in this one, too.
Kotonoha - I don't know how to talk about this one. IT'S JUST REALLY GOOD. AND ALSO THE INSTRUMENTALS ARE PRETTY. Like, the exact kind that I like.
Mintblue Girl - it keeps coming on in the car and when I'm playing Minecraft... it's just REALLY GOOD!
citron - I kinda like how everything sounds so high in this song. But, what I like about it the most is during the chorus where the instrumentals rises in intensity for that one second before she finishes it off in the vocals.

La prière - a vocal group consisting of her, Natsume Itsuki, and another singer named nayuta. Itsuki (short yellow hair) has the high-pitched voice. nayuta's (long black-purple? hair) voice sounds very similar to Nakuru's, but subtly she feels more mature.
Triptych Symphony - WHY IS THIS SONG ALLOWED TO BE THIS GOOD!?!? Anyways... my favorite part is the instrumental break. THAT KEY CHANGE HITS SO HARD!?!?!
Necrogamy - the first half sounds like some sweet wedding song BUT THAT IS NOT IT AT ALL!? I love it when stories are both nice and dark at the same time. LISTEN UNTIL THE END!
Evo - most relatable song I have ever found (I have a 1100+ songs playlist of just Japanese music), aside from the one where main character finds out he can't become a cat. Both of these songs are about the same subject by the way... anyways LISTEN and watch the MV! This one is composed by Chinozo, who is quite famous in the Vocaloid world for some of his other works.
点心爛漫！ふぁんはおれ - I genuinely don't know how to translate this into English. English doesn't have quite the same things. ANYWAYS, WHY IS IT JUST STUCK IN MY HEAD!?!?!?!? WHY IS IT SO GOOD!!!?!?!
Susume! Lovely Idol La prière - finally a song where they are just being cute idol girls! This one is very bubbly and they're all singing cutely too. "Susume" means "(go) forwards".
キボウノホシ - "the star of hope". April fools song where they pretend to be guys instead, except it's REALLY GOOD!!!!! I promise.
それは世界を越えて - "that (which) will surpass the world". GO LISTEN TO IT IT'S REALLY REALLY REALLY GOOD!!!!!!!!!!! It keeps coming on in the car... the chorus is like, so fire.

I have no idea why I wrote this much. I'm honestly more passionate about band and orchestra music. You can see how my comments get more personal and emotionally charged as the list goes on...