Rarity seems to be the obvious choice here, as she has been shown to be an overreactor in the past who feels things intensely and with passion. If we think back to the episode where the crew go to the galloping gallor, Rarity goes gooey eyes and throbbing heart for the Prince character whose name escapes me; the one who turns out to be a complete jerk.
In regards to the other ponies, we can deduce the following:
. Twilight was shown to have an interest in Flash Sentry in Equestria Girls, but did not melt into a puddle over her attraction for him. She merely blushed and stumbled over words, which was adorable.
. Applejack tends to conceal her feelings and not make them known. She also comes across as slightly stubborn so I doubt she would melt into a puddle either, or at least not when others are around.
. As for Rainbow Dash, although her emotions come out more than Applejack, I believe she thinks herself too awesome to be fawning over a male and would likely conceal it. That being said, she does make her adoration and awe for Daring Do obvious, but this is different.
. Fluttershy is...well....shy, but I wouldn't put it past her to also go gooey eyes and melt in a puddle; she would just do it very quietly and in her own surrounds.
. Lastly, we have Pinkie Pie. I am unsure as to whether she would melt in a puddle to be honest. I believe she would more likely be upfront with her feelings and probably go straight to asking the male out, due to her confident nature and outgoing, charming personality.
But hey, that's just like, my opinion man.