I wasn't sure how to vote since I couldn't vote for more than one.
The 12.9 and 7.9 are the iPads that work best for me. There is of course overlap in their functions, but these are the functions that are important to me in which each device excels uniquely by a significantly high margin (among my devices at least, but in most cases of any device):
12.9 - very portable digital drawing, always in my bag laptop replacement, very portable content immersion, comics and illustration viewing, portable secondary display for my laptop
7.9 - ereading, taking notes under the radar, super portability around the house for general iPad use
But I think why I choose these iPads makes more sense when looking at all my devices. To complete the picture:
iPhone SE - always with me and unobtrusive communication and computing, the only true one-handed device. This is like my tech pocket knife.
15" MBP - (portable) work station for heavier duty macOS software
21" Cintiq - bigger screen drawing at my desk
Mac mini - always home computer, media hub, central file hub with time machine backup
This is the most ideal setup I believe I can have right now. But this is because of my job as a media artist, which is somewhat demanding tech-wise. If I had a less techie job, or one I could completely separate from my personal life, I would probably have the SE, a 7.9, and a 12" MacBook or 13" MBP. Since in this case the only iPad I would have is the 7.9", I ended up just voting for that.