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Everyone will have a different opinion about this. And we ourselves are probably constitutionally incapable of picking just one.
Must-sees, in our opinion, include: the Fremont Street Experience downtown; the pirate-siren show at TI; the water ballet at Bellagio; the optical-illusion marble floor at the Venetian; the views from the top of the Stratosphere, Eiffel Tower at Paris, VooDoo Lounge at the Rio, and Ghost Bar at the Palms; and the Atlantis animatronic show at the Forum Shops at Caesars.
In fact, why don't we put it out there to the QoD faithful in a poll? If you had to pick one and only one must-see attraction in Las Vegas, which would it be? Email us your selections and we'll post the results in an upcoming answer.