I think of all the gadgets in the books I would want these items for the following reasons:
Weasley's family clock - that clock is the shizba. You'd always be able to do whatever sneaky thing you wanted and would know when that someone you don't want to know what you're doing comes home (great for surprise parties btw)
Firebolt - face it. Everyone had dreamed of flying at some point.
The jar with Rita Skeeter inside it - I want to be able to contain that woman. Better yet, let's change Umbridge to a toad and put her in the inescapable jar.
blaugh