3. Transslammers - The Jam - Quad City DJ's vs Stan Bush
This is the mash-up which inspired this list. Almost surprising that it's not #1, really, but there are two reasons for that, one of which we might come to regret later once some buzz wears off, but we'll see. In any case, both Stan Bush's "The Touch" from the original 1986 Transformers movie and the Quad City DJ's "Space Jam" theme from the 1996 movie are rather similar, with the whole motif of rising up against an adversary going on, so this decade-spanning mash-up manages to work rather well, perhaps even surprisingly so. The alternating of lyrics back and forth also works pretty well, giving both songs plenty of time by letting Space Jam support the main song, which is always a plus when done well.