The Lansky Puck tool sharpener. Got it as a gift a few years ago. I’ve stoned fresh edges on everything from Swiss Army Knives to spades with this thing. Speaking as an idiot, and someone who has not got on with sharpening devices in the past, it would appear to be pretty much idiotproof. Proving I’m an idiot, I almost took off a fingertip with a spade edge after just two passes on it with the Puck, and that spade hadn’t been sharp in seven or eight years.
They seem to be only available in pairs in the US, for $24, but you can buy them singly in the UK for £15. This link should work for most places.