I almost want to say A Canterlot Wedding, because it makes the most sense if you stick it near the front. Maybe right after Friendship is Magic part 2.... There would be no confusion as to the sibling that never comes up.

But honestly I think Ticketmaster is best, simply because Friendship is Magic Part II is so... Simple. You know who is what element and that they are the elements before they even leave the library (the whole sound effect while blinking thing is jarring!) Part I is actually much better IMO, and does a wonderful job of setting up the story. Part II needs to be remade into a movie or something, so it gets past the "each person does a token example of their element" factor.