I think it would have the be the rarest legit one.

Technically you can get tank kills on bluewater ships, but that requires custom games to ever get so I probably wouldn’t count it.

Edit: High-calibre TD kills might also be comparibly rare nowadays - things like the Bkan than can’t aim upwards. It used to be suprisingly easy to kill planes with them, but since the HE changes years ago, they’re pure RNG.