Well, somepony on "Padded Ponies and Adult Foals" suggested I do this, so here it is. I should mention that when I say princesses, I'm only going to include three of them, Celestia, Luna, and Cadence. I don't think anyone will object to Flurry Heart being left off, since she's still just a baby so it's unfair to speculate what she'll be like as an adult. As for Twilight, I already covered her under my mane eight ranking, so I don't want to be redundant by including her here. If she DID, she'd probably be number 1. Now, as with the others, this list is not official, it's based on my observations. And with that out of the way, we can begin.

3. Princess Cadence: Honestly, it's hard to really say for sure who's less likely to be padded between Celestia and Cadence. Both of them are much better at being motherly figures or caretakers, because of their personality, behavior, and yes even maturity. Prior to "A Royal Problem", I'd have probably swapped the two, and I'll explain more about why Celestia is in second when we get to her. Now, as for Cadence, I think the biggest arguement you could make for her as an AB/DL, would have to be either wanting to give Flurry Heart a playmate, or helping Shining Armor learn how to handle a foal. I suppose part of the reason why there's so few padded fics of Cadence, is specifically because she's so motherly, and also seems pretty content with her life. I could use some of the comics and expanded media, but that would be cheating as that stuff is all secondary canon (it's canon unless it contradicts the show).
2. Princess Celestia: "A Royal Problem" showed that Celestia's job is very stressful and demanding, to the point where she can find it hard to smile. As a result, I could see her perhaps possibly becoming a closet AB/DL as a way of relieving or coping with stress (sort of like "Even The Sun Needs A Break"), but she would probably need to be introduced to it to even consider it. And I could easily see her feeling guilty and unwilling to indulge, even for her own benefit. Like with Cadence, Celestia definitely works better as a motherly figure or caretaker for someone who might be padded (which is probably why there's a good deal of padded fics involving Momlestia), but I think "A Royal Problem" made a stronger case for bumping Celestia up in exchange for knocking Cadence down.
1. Princess Luna: Even prior to "A Royal Problem", it was clear that Luna still has some hidden insecurities and issues, either with her sister or with her own past. That's not to mention the fact that she was originally just a filly when she came back, so it's possible her time on the moon could've at least temporarily affected control of her bodily functions. Assuming she didn't need to wear diapers (and she probably doesn't anymore), Luna would most likely become an AB/DL to deal with the fact that she still harbors a lot of guilt over her past, but also has no one to really talk to for advice. She probably would try to hide it from Celestia, and her subjects, but she likely wouldn't be nearly as paranoid about it, or feel guilty. Luna even seems like the sort of pony who could benefit from some pampering, as one could argue it could be a way for her to let off stress without scarificing sanity.

Well, there you have it. Feel free to go ahead and discuss how you'd rank the princesses, you can even toss in some of your headcanons for Flurry Heart as an adult if you want to.

You could have included Flurry Heart in the number 1 slot, and just said "She's a baby, so she doesn't get a choice" :-p

There seems to be a thing in a lot of fanfics where Cadance's royal duties include practising every fetish imaginable. If that's true, maybe she's tried the sexual AB/DL thing before. And I know it's not canon, but it's easy to imagine her as the kinky one behind closed doors.

5983604 Perhaps, that's open for debate. I think putting Flurry Heart at number one isn't fair, since we're talking about Adult Baby, and we don't know what sort of adult Flurry Heart is going to be.

I don't agree with your point of view about Luna. You say it's hard to imagine Celestia and Cadance padded for being too motherly. But the truth is that throughout the show; Luna was the one who behaved more maternaly of the three. Showing a particular concern about kids, advising them about their fears and actions; Being also the only one of the princesses that actually taught lessons directly. In addition the Princess Luna proved to have a very severe character even more than that of Celestia.

Actually. I imagine Cadance more like a switch that a 100% mommy. Many mommies are abdl too and this can be seen on Cadance in her carefree and slightly child like behavior that she had when was babysitting Twilight. Twilight also considered her as her "sister".
I can imagine more scenarios in which cadance can be an abdl. There is something called Electra complex in which a girl feels the need to rival her mother for the attention of her father. It would be very lovely and adorable to see Cadance and Flurry Heart competing for the affection of Shining Armor as two babies sisters. lol