I always thought it was a prop, but then again, I may have confused it with that old AT&T add, the one with the fax machine on the beach. Some nifty props in there too.
Doc Brown's DeLorean time machine, hands down. It's my dream to one day buy a DeLorean and make it up to look exactly like the one in the movie. Proton Pack - Ghost Busters (I'm eventually going to build one, if I can find good specifications for one online [not an ACTUAL proton pack, but a prop])
Constable Odo's bucket from Star Trek: Deep Space Nine. The Stargate from Stargate & Stargate SG-1. Doc Ock's arms from Spider-Man 2. Luke Skywalker's green lightsaber from Star Wars: Episode VI - Return of the Jedi. Admiral Al Calavicci's handling from Quantum Leap. Those are the first to come to mind...
A Klingon Bat'leth. That thing with the red lights which move back and forth but has no discernible function of any kind. It's at 1:45 here:
The Star Trek Hyposprays. God, I hate needles....... The costumes in Logan's Run. But you need a hot body to wear them.....
For "hand" props, TOS Phaser 2. If full-sized vehicles count, the 1966 Batmobile. If shooting models count, the Harper Goff Nautilus.
I have often wondered what the hell that thing is even supposed to be besides a cool light? It's appeared in a lot of shows with few modifications. In fact I'm pretty sure it's been in Star Trek too.
Speaking of non-functional moving lights, this gizmo has spent plenty of time in front of a camera. I'v always thought it would be cool to own that thing -- assuming it hasn't been lost or destroyed by now.
It's hard to pick a favorite. There are so many that I love - most of which have already been mentioned. But if I had to pick one, it would be the DB-5. (Funny enough, GF is one of my least favorite Bond films.) Though as far as "best," it's pretty hard to argue against the ruby slippers.
Just thought of one: the fossilized Creature hand from THE CREATURE FROM THE BLACK LAGOON. Or maybe the wolf-headed cane from the original 1941 version of THE WOLF MAN?