I'm surprised that they haven't parodied the Friends opening yet, it seems like the most obvious choice for the following reasons;



1. It's a show with a friendship based name and a friendship based theme song.



2. Bronies, pegasisters, and ponyfans have been making Friends related jokes, parodies, and fan arts of this show seemingly since it began (this picture included.)



3. Friends and My Little Pony: Friendship is Magic have had at least one similar joke (a game of staying quiet, which the loud character(s) promptly lose on purpose.)



4. They have named one episode in the Friends style ("The One Where Pinkie Pie Knows".)



5. It is a still popular show which still airs, and while there's nothing really stopping the actual target audience from getting the references to the other shows they've parodied, Friends is the most likely for them to get as it would be easiest for them to know about it, and be fans of it.