The song is from season 2 episode 26 of MLP:FiM.
Cadence Aria: http://youtu.be/QhlLst225Ls | ♫ Lyrics for the song:
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[Queen Chrysalis]

This day has been just perfect
The kind of day of which I've dreamed since I was small
Everypony I'll soon control
Every stallion, mare and foal
Who says a girl can't really have it all? [chuckle]