3. Blink To The Future (Doctor Who/Back To The Future)
This momentous mashup - aptly titled 'Blink to the Future' (we see what you did there!) - brings together the coveted universes of Doctor Who and another timey-wimey franchise, Back to the Future, for a crossover that really should've happened by now. And somewhere in the jaw-dropping vastness of the multi-verse, it probably has.