Thought most of them were complete crap until one of my Facebook friends shared this haunting pairing of completely unrelated songs -- "The Boys Are Back in Town" by Thin Lizzy and "Monkey Gone to Heaven" by The Pixies. My seal of approval for a mashup, besides if it rhythmically fits, is given if it causes you to appreciate the original songs more.
Then that cursed YouTube, always at the ready to suggest something related to what you've been searching lately, pointed to another Pixies mashup, "Where is My Mind?" coupled with The Beastie Boys' sadly overlooked War on Terror commentary "In a World Gone Mad."
There's several "Smells Like Teen Spirit" mashups, but the one that makes the most sense is this one with White Stripes' "Seven Nation Army," Seven Nation Teen Spirit, if you will
Then these next two were so good -- because of the polar opposite juxtaposition of overt '70s pop with '80s hard rock -- they brought actual tears to my eyes.
STAYIN' IN BLACK