The player.

If you're talking about the actual properties of the instrument, if I had to narrow it down to 5 things I'd say wood types, scale length, hardware quality, a proper set-up and construction (bolt-on vs set neck, etc). Maybe not in that order.

The amp and speakers are just as important as the guitar. A low-quality guitar through a good amp rig will sound better than a good guitar through a low-quality amp rig.