Wingtips, for sure.
Many an expert fighter pilot has creamed my Lancaster with very little ammunition by striking from above, and fast, and dinging the wing tip with a few shots. There's no way a buff can make it back, and once the spin starts, all you can do is bail out and pass along a .
Wingroots are good targets too, but those seem to require more hits, and I've made it back with all kinds of damage to the fuselage before.
If you are looking for the secret to using as little ammo as possible with the best results, as mentioned, wingtips.