No. 10, where I could cross out 'scrap Ark Royal and Harriers' on the PM's job list and write, 'buy extra ships, Harriers and CSB and scrap the Crabs'.
Assuming I've got an invisibility cloak or somesuch, I'd like to get in a bank vault and make away with a load of dosh and diamonds etc. If it was only me that was invisible, the piles of loot would look a bit suss floating across the floor towards my waiting motor.
Having sorted out the UK's defence problems and now minted for life, I would then have to make my way to the bedroom of a sleeping Agelina Jolie, where I would have to stop myself giggling when she receives an invisible faceshot and wakes up wondering why her mouth tastes funny and her hair is matted