I think MLP has quite a few... Interesting ships. I think we're all familiar with the Mordaci X Twilight ship, or the Vinyl X Soundwave, ad of course there's the Pinkie Pie X Dan ship, but what about the other ponies?

Who is the crackship for mares like Applejack, Luna, Celestia, Rarity, etc.

Surely there is always more than one crackship. If you need another, you can just make one up on the fly.
But for Rarity, some people think the crackship is Sparity.

If you want crossovers, I've seen Celestia shipped with Superman.

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Optimus Prime x Princess Celestia.

Rainbow dash x sonic is one I’ve seen pop up pretty often

I thought Tempest and Neighsay is a crackship to me, but I think there is reason why these two are shipped.

crackships you say

This just reminds me that I once wanted to write a massive crossover MLP story for the sole reason of shipping Marceline from Adventure Time and Wendy from Gravity Falls. XD

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this :D