What's your favorite background in Cave Story?

In what may be considered the SERIES CLIMAX of the bottom of the barrel "What's your favorite _ in Cave Story" threads
(unless we somehow come up with more ridiculous obscure ones but ngl that just sounds like it'd spiral into drama unless it was limited to in-game mechanics / code exploration stuff)
we have the ever important question ::.:
The shit in the data directory that is prefixed with bk

Here's your contestants:

This is the most important election in the history of democracy or something like that idc this shits funny asf

 

I'm guessing bkMoon would be a popular choice, and it's pretty, but I actually really like bkBlue. it's simple, not overbearing, gets the job done and is easy on the eyes.
I've always kind of wondered how these backgrounds work, and this is hard to put into words, but like, most of these backgrounds are not very obvious to interpret as an actual physical backdrop (save a few). So what exactly are we seeing when we see these backgrounds? How did Pixel think of making these patterns exactly, that blend in without us questioning it and appear natural despite not really 'making sense' in the sense of representing something? Thanks for making this thread because I have wanted to ask this question for a while now, actually, and this gives me a chance to do it.

 

I'm a big fan of all the closed-in areas that use bkBlack. At first I was going to say this as a joke but I think I still agree if I think about it seriously, it's an atmosphere thing
First cave is iconic for a reason, it's like comfortably pinned off, esp the save room and the one where you get Polar Star