4. Laser Screwdriver
The Master's nifty gadget is as terrifying as it is awesome. It doesn't just use laser technology to fire beams of energy at whoever The Master targets, but it can also artificially age (or de-age) the target - providing that the device has the target's genetic code. Not only this, but the controls are isomorphic, meaning that it can't be used by just anyone. A sign of how ingenious and also how paranoid The Master is, the controls stopped The Doctor threatening his Time Lord nemesis on the aircraft Valiant.
Used by John Simm's incarnation of The Master, the laser screwdriver was created in a way to mock The Doctor's own sonic screwdriver, with The Master jesting "Who'd have sonic?", and it was notably used when The Master killed his future self, Missy, when she turned against him and chose to side with The Doctor. Although it wasn't The Doctor's gadget, it really was an awesome piece of tech, used by an even more awesome villain.