To my mind there is no perfect solution to the "spare" problem.
On the back-door - Knackers the hinges and the doorframe and obstructs access to a hitch - especially NATO hitches or needs a swing-away mount (??).
On a roof-rack - A beggar to shift and moves the Centre of Gravity up, uses most of the recommended weight allowance too and needs a rack (??), which makes it even harder to fit in a car-park.
Inside the vehicle - reduces usable storage space, guarenteed that when required it will be hardest to reach !.
On the bonnet - Heavy to lift, some reduction in vision just in front of the vehicle.
On the whole, I prefer mine on the bonnet. I'm used to it being there - and it looks "right".
MOD took H&S to heart when spec-ing the WOLF, which comes with a side mounted spare held in a fabric sling arrangement that lowers the spare to the ground. This mod adds 6-9 inches to the effective width of the vehicle and requires extra long mirror arms so you can see past it. A line I heard was that this was because otherwise a bonnet mounted spare hits the top of the engine on a 300 Tdi - which I doubt, mine certainly hasn't in several tens of thousands of miles. More money wasted on "gold-plating" in an MOD contract