I mean, they all seem to love foals and will care for foals that aren't even theirs. Twilight might like the idea of being a mom, but it'd be hard for her to pursue a relationship and it'd probably make her fairly anxious. Flutters is the most motherly in terms of personality. Rarity is rather strict to Sweetie, but she doesn't dislike foals. But would she want one for herself? She's interested in romance, so she's likely considered it. AJ is big into family so she absolutely would want children. Pinkie loves foals and is, canonically, the only one who's a mother. (The only part of the finale I liked, btw.) Dash also loves foals, but she needs to take a few levels in maturity before she's ready.

Above: One of the few good ideas the writers had for the finale.

Also, Pinkie had sex. Let that sink in.

Fluttershy. She’s the element of kindness, but knows when to be stern. That is what should describe a parent.

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And she's good with kids too.

Exactly.

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Twilight Sparkle
Her being the ruler of Equestria, I don't see her engaging in romance anytime soon as much as Celestia and Luna canonically did. For her to have her own foal, there must be somepony or creature she is interested in partnering with. You could argue it being Sunburst since the two built chemistry together in "Uncommon Bond". Or maybe Flash Sentry...maybe (their relationship never developed any further besides being a crush. There's no real history to it compared to Pinkie and Cheese). However, I see Twilight inevitably sharing the idea with Celestia where she will view all her ponies as her children (basing that from how Celestia would call her citizens her "little ponies).

Rarity + Rainbow Dash
I can't really see Rarity and Rainbow becoming a mother. Those two appear much too married to their job to ever settle down since their job forces them to travel a lot. Yeah, Rarity would likely spark relationships with stallions, but unless that stallion is willing to become a stay-at-home dad or at least remain at a single place to raise their foal, I don't really see her tying the knoose.

Fluttershy
I agree that she would be most likely to become a mother. I'm actually surprised that Pinkie Pie had one before her.

Applejack
Yeah, I know for a fact that she's going to find somepony eventually. She would like to continue to the family bloodline. I could probably see her having three foals just like her mother, Pear Butter.

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I feel like Dash would channel her latent maternal desires towards coaching foals, like with Scootaloo. Not sure about Rarity-by the end of the series Sweetie basically was her daughter.

7194087 She said it herself: "Fluttershy: Good with animals, good with kids"

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What was the arrangement of Sweetie's residence with Rarity? Does she mainly live with her sister? If so, why? I mean, it definitely had to be the case since Sweetie attends the school at Ponyville, right?

In the episode of the Sisterhooves Social, I recall their parents stating to have Rarity take care of Sweetie Belle for a week while they went on vacation. That would imply Sweetie was still living with them, wouldn't it? I think the question to be asked here is what changed? Or, where do their parents even live? Do they still live in Ponyville (Rarity was born there after all)?

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Fluttershy already has a kid.

His name is Discord.

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Honestly, I always pictured AJ wanting to be a parent the most ... only way she wouldn't have a few is if she was having fertility problems or trouble finding somepony to settle down with. Pinkie ... well, I don't think Li'l Cheese was exactly planned, but very much welcomed. Flutters probably wants a foal or two, but might also run into the lack of partner issue, especially if Discord is scaring off her suitors. Rainbow Dash probably would just prefer to coach or foster once she retires, but might adopt. Rarity might, but only on her terms, which would almost certainly involve not being the primary caretaker. As for Twilight, probably not, and if she did, it'd probably be with donor sperm and/or fertility magic.

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Simple for me. Fluttershy. For all the reasons other people have said. If she could find a stallion and work up the courage anyway.

As for the rest of the Mane 7.

Rainbow: Far too immature to be a mom and raise her kids right.
Rarity: Married to her job I feel.
Applejack: Also too immature to be a mom and raise her kids right. Even if she wasn't married to the farm.
Pinkie is married, so no comment there. Still shocked she is though. But that's just me 😁.
Starlight: I can see her being a great mom. If she ever found the right stallion (and I refuse to believe that she and Sunburst are a thing).

Twilight is... Harder to work with. When you're God Empress of Equestria, that's not a lot of time to find love. Let alone have a family and raise her children right. But I do believe she would be a great mom. Not a perfect one. But Twilight would make the time for her family, and make sure her children knew her as their mom. Not the princess they barely ever see.

And who is going to tell her no? Just a thought though.