Between any of the Main 6, or the Equestria Girls, or anyone else like the princesses, sirens, Silverstream, Ocellus, etc.

For me... it's probably a tie between Rarity, Twilight Sparkle, and Pinkie Pie. (I cannot get enough of them farting) Rainbow Dash and Celestia not far behind.

As far as humans go... Mostly Sunset, Twilight, and Rarity, but only slightly more than Rainbow Dash, Fluttershy, Pinkie Pie, Sonata Dusk, or Aria Blaze.

Of course we never see it in the show (but we do see Rarity and Fluttershy fart in the comics), but that's what fanfics and fanart are for.

Celestia and twilight farting on spike

Rarity
Definitely Rarity
Because she's best pony x3

Cadence, Luna, applejack & Rainbow dash

7072690
Yeah!

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Which comics?

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In Issue 3 of the Nightmare Rarity comic (Issue 7 in total), Pinkie makes reference to Rarity farting in the tub at the spa while laughing at one of her jokes (to which Pinkie mentions Rarity was as embarrassed about it as you can imagine), and it is shown in a flashback in Issue 8 when Pinkie remembers it as her fondest memory of Rarity to help save her from Nightmare.

In Issue 73, Fluttershy falls under a spell making her imitate many animals. There is a moment where she is imitating a skunk and walking beside it past Rarity, who is disgusted at the smell. It's not made clear whether the smell was from the skunk or whether Fluttershy was imitating the skunk spraying someone the only way she could, but given that the comics already depicted Rarity farting onscreen, the latter is not unlikely. (Rarity forgets about it immediately after as if it was no big deal, never referencing the event again before or after finding out that Fluttershy was under a spell)