I can't tell which YouTube videos are being linked to since I can't access YouTube at work, so apologies if they're already mentioned...

If you're talking about marathon medleys, South Park's La Resistance is amazing. It's catchy, it's funny, all the songs work together beautifully. Sure, it's crass, but South Park is always that way. It's amazing because of [i]and[/] despite the crassness!

If you're talking mixes, then Prodigy/Enya's Smack My Bitch Up / Orinoco Flow is awesome. I'm not sure if it's better than either song individually, but I'm not sure it's worse than either song individually, either - all three pieces are great!