The one (listed under other problems) which always struck me since the first time I heard it:
"How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse or quantum decoherence, give rise to the reality we perceive? "<p>I still vividly remember the first time I heard the "collapse of the wavefunction" and openly challenged our poor Solid-state electronics professor - it led to a looong discussion at the end of which he got flustered enough that after shifting thru entanglement, decoherence, Measurement problems, many worlds interpretation, Bohm, time travelling interactions, philosophy, etc he pretty much kicked me out of his office.<p>I think I might have partly contributed to him seeking a transfer to another dept. the next year...