This was a really hard one for me, I love them all but was very tied between Waterfire and Strawberry Reef. (The only other one I have is Island Rainbow, but she is far from my fave pony to be honest, lol.)
Waterfire was a childhood pony of mine, and I love how unique she is. She also has flames in her eyes instead of the regular star or circle! I love ponies with unique eye glints. She's also such a beautiful blue. (And you guys are right, she totally looks similar to Barnacle! I feel like they'd make a pretty cute nautical pony couple.)
But Strawberry Reef is also super cool, despite being my least favourite colors, her name and pose are pretty funny and I love her symbol. She is also sentimental to me as she was given to me by one of my long time best friends, who also used to collect ponies with me.
I ended up voting Strawberry Reef, but honestly it was hard for me to pick one or the other. Now I'm thinking I should've voted Waterfire, but I'm sure if I did I'd be thinking the same thing about SR...