For starters, I don't consider any of Ado's song bad. Some aren't quite to my taste, which is mostly what dictated their placement on the tierlist, so it is pretty much entirely subjctive. If you disagree with a placement, that is entirely fine; I would consider most of Ado's lineup to be worthy of being someone's favourite song.

I have heard every one of those songs dozens of times, most of them more, so at this point, I have a pretty good idea of what I want from an Ado song, if that makes sense. So while some songs might get a bit overplayed for me and I will prioritise others, I'm quite certain that the tier of a song will not change for me anymore.

No song of her is below a C-tier for me, which would be around the enjoyment I would get from a song on the radio that I don't particularly enjoy or dislike, that won't make me switch the channel, but is enough to make me skip it if it comes up during in my playlist, unless I'm in the mood for that song. The tiers are ordered, so songs that are further to the left are songs I enjoy more than to the right.   
I used the studio versions for this list, because while I tend to enjoy live versions more, Ado changes the way she performs certain songs quite a bit between performances, and I would just be stuck in a quagmire of trying to figure out which one I like the best. (But yes, Aishite Aishite Aishite would be in S-tier if used live performances, you can put the knife down) I tried to rank the songs by themselves, without considering external aspects that influenced the creative process, but for some, that isn't possible. Vivarium and Shoka would always rank near the top, even if they were only A-tier (they are still S-tier tho), for the simple fact that they are made entirely by Ado, and for me, that is something I value.  
  
I used an existing list on Tiermaker, so if one of her more obscure covers is not on this list but you want to know what I think of it, just ask. I'm also open to friendly debates about why any particular song ranks where it does, of course.

(Yes, I know it's not Tuesday.)

As I made it into the top 0.01% last month, I felt obligated to share a tier list with you (please read the caption)