The song is from season 1 episode 3 of MLP:FiM.
♫ Lyrics for the song:
----------
[Pinkie Pie]

Twilight is my bestest friend
Whoopie, whoopie!

Twilight Sparkle: Pinkie...

[Pinkie Pie]

She's the cutest, smartest, all around best pony, pony!

Twilight Sparkle: Pinkie.

[Pinkie Pie]

I bet if I throw a super-duper fun party, party!

Twilight Sparkle: Pinkie!

[Pinkie Pie]

She'll give her extra ticket to the gala to me!