Still my favourite mashup, I think... Stayin' Alive In The Wall (Pink Floyd vs Bee Gees Mashup) by Wax Audio
Although this is also rather good
Bob Marley vs The Beatles - Let It Be, No Cry - Mashup by FAROFF (No Woman, No Cry and Let It Be)
And this has a certain something, not as obvious as many...
ABBA vs Soft Cell - Tainted Waterloo
I have posted mashups before; do you know of any others that are really good?