What’s the best and most convenient connector for 12v appliances onboard?

The boat has some of these:

They’re often called ‘Clipsal 12v’ sockets, (Clipsal makes a wide range of electrical gear); sometimes called a ‘2 pin parallel’ connector. I suspect they originate from caravanning. Note the neutral blade is wider than the live, for a polarised connection. Not to be confused with what I think is an older caravan 12v connector often called a ‘W4’, where the blades are the same size and angled:

Both of the above can handle 10amps. Other common 12v connectors found are the ubiquitous cigar/cigarette connector:

Many 12v appliances come with a plug to fit they above. They’re not great connectors because they are an adaptation of the cigar lighter, and were not designed to be a DC connector.

I’ve also used a 12v ‘Hella’ plug (a nickname after the German brand that makes them). I first encountered this on our 1975 John Deere 2120. Hardly any tractors at that point had any kind of 12v outlet; it was probably fitted because the tractors were German made (Mannheim), where this connector is popular. It’s quite a good connector, but only 8amp (at 12v; can also be used for 24v)