Well there are so many different gadgets to choose from, I can only narrow it down to the top 7:
The escape watch
The toaster
The golden parachute
Underwater talking gadget*
Ruby's bike from the 6th book (it has special effects and is made by Hal so it counts)
The fly clip
Hitch's silver fire-suit-thingy
*this gadget is only vaguely mentioned in the beginning of the second book when Ruby is learning hand signals from Kekoa and she asks if there is a gadget for underwater talking and Kekoa says something along the lines of "there is, but you don't need words down there, better this way, there is a signal for anything you will need to say"