I did this list originally over four years ago. In the time since a lot changed, at least for one of the fillies (not going to spoil who). New developments in the show's continuity and canon prompted a re-evaluation of this list. I could probably also add the likes of Gabby and Babs Seed, both unofficial sixth rangers of the CMC, but then again there are a number of other candidates with noticeable one-off interactions with them that could qualify. Anyway, here's the original if you're interested.

5. Silver Spoon: As I said in the original list, the best thing Silver Spoon has going for her in this is that she's a follower. If somepony she admired, looked up or was close friends with got into diapers and introduced her to it, she'd probably go along with it. That's really the only thing that can be said about her since otherwise she is a completely blank slate. We have no clues about what her home life is, how she met Diamond Tiara or even what her special talent is meant to be. You could make an argument for her being anywhere on this list because of her being a follower. But on her own there doesn't seem to be much that would compel her to seek it out.

4. Apple Bloom: True, she absolutely hated being called a baby and even more so when she was actually treated like one by Applejack. She had a really bad experience which would probably go a long way toward turning her off from the world of AB/DL naturally. But there is one possible avenue that might work. "The Perfect Pear" made it clear that she's too young to really remember her parents (Applejack and Big Macintosh at least had to have some memories), though they didn't say when exactly they died. Losing parents at a young age can have profound impacts on a child's development and mental state, so it's possible that Apple Bloom could get into it at all as a way to remember a more carefree time. A time when her parents were actually around. She'd still be extremely afraid of being caught and would go to great lengths to hide it, especially from her family regardless of how they might react.

3. Diamond Tiara: She's staying the same. It does look like at the least she was happy after her redemption in "Crusaders of the Lost Mark". But we know Spoiled Rich was still hanging around and hadn't learned her lesson, so there's no telling how well the family situation really was. Everything we learned prior still applies, particularly Diamond's lament about how her whole life was on dictated terms until she finally stood up to her mother. Jealously and a desire to have a proper childhood could easily be an avenue for her to get into AB/DL, and who knows how well she'd take to it or who she might trust with protecting the secret.

2. Scootaloo: Now that we got to learn more about her exact family situation due to her aunts and her parents, it provides a pretty big boost for her to want to be an AB/DL even just a little. Yes, I'm probably a bit biased because I wrote two padded pony stories featuring just such an idea, but the idea is there. It's not clear how long her parents were away or how long she was in the care of her aunts (it has to have at least been since Season 4 given what we saw in "Flight to the Finish"), so maybe her parents were around more in her early years and only recently started traveling. Or maybe even as a foal she was cared for more by her aunts than her parents. Even so, despite her initial dislike of being called a baby by Babs Seed, there's probably a reason why there's so much padded art of her (complete with an entire label for it "Crinkleloo" originally based around an "Ask X" web comic).

1. Sweetie Belle: More padded pony stories involving her as the baby have begun cropping up, and a lot more padded art of her has started appearing now. Still nothing really as a result of "Forever Filly", but there has been stuff with Rarity. There isn't anything with her parents though, surprisingly. And I feel like that might be an avenue to explore. What was Sweetie Belle's childhood like, especially in comparison to Rarity's? Were her parents just as hooves off then as they appeared to be in their only major canonical appearance, and Rarity did most of the child raising for them? Or were they around more often and took a more active role in raising Sweetie Belle themselves? If so, how did that compare to their approach with Rarity? Even as she got older, Sweetie Belle never lost that childlike innocence, though strangely enough her somewhat ditzy ways were dropped after "Sisterhooves Social", not that I'm complaining. She still easily retains the top spot. It really is a competition between her and Scootaloo for most diaperable filly.

And there you have it. Let me know what you think of this updated list. Maybe there's something I didn't consider that you did.