I adore Cicierega, and if somebody hasn't listened to his Mouth albums, they should rectify that, but I think maybe my favorite mashup is
Isonine's We are Coming Undone, which is KoRn vs Taylor Swift (Unfortunately, looks like it's been removed from Isonine's channel, but here is another upload from somebody else). It's just so damn good.
I also really like The Hood Internet's Justin Timberlake x Daft Punk (Suit & Tie x Get Lucky), which, if I remember correctly, only had the teaser version of Get Lucky in it because the full song hadn't been released yet (could be wrong on this)