:O This is a good question.
Of course with how obsessed I am with them it can't just be ONE song, so I made categories so this can be extra fun!!

Favorite with them both together- https://www.youtube.com/watch?v=sSYoz0JmnZo
Electric Angel (originally used Miku)
I find this song very nostalgic and it was the song that really made me fall in love with both of these characters! It started a long, long obsession... I actually thought that this was the original version of Electric Angel for a while, but it isn't.

Favorite solo Rin cover- https://www.youtube.com/watch?v=oTxyT31u4Kk
The Vampire (originally used Miku)
A more recent song than anything else on the list but this song is an absolute JAM and I love its kodocon-y theming

Favorite solo Len cover- https://www.youtube.com/watch?v=QqfZ9s-uUK0
Unravel is originally the Tokyo Ghoul opening theme
This one was hard... there's a lot of good Len covers!! I had to go with this one though because it's one of my favorite anime OPs sung by one of my favorite boys. What's there not to love? Also apparently there's a few people who have done Len covers of this song, but this one has always been my favorite since I first heard it several years ago! His voice just has this emotional quality that makes him sound like he's about to cry, which is something I really value in a song I guess lol!

Favorite famous song - https://www.youtube.com/watch?v=eSI7RsjZy1E
A classic and a bop! This song deserves all of its fame. I love playing it in Project Diva and Sekai, also.

Favorite niche song- https://www.youtube.com/watch?v=B5s7191duYc
Girl A is sung by Rin, but I didn't know that until I'd already listened to it, possibly hundreds or thousands of times. My fellow Vocaloid otaku will immediately hear the producer name siinamota, and feel a clench in their gut. A beautiful soul and talented artist lost far too soon, we can hear the pain he was experiencing in his songs, the last few things he gave to the world before it became far too painful. All of these songs are really close to my heart as a depressed person, and are a reminder to keep going no matter what, because there will be people who understand, and people who will mourn you. You simply haven't met them yet, and it shouldn't be your loss that introduces you to them.