It seems that the widely accepted "best" socket in the work is the UK BS-1363 (type G) socket. This is a pretty decent opinion, but I personally think it is the second best socket in the world. Here is why:
 - Ground prong is the same width as the live/neutral pins, just longer and unshrouded. This means that with a cruddy noncompliant socket that doesn't have a wide enough flange (common with international sockets), you can insert the ground prong into the live part of the socket, energizing the chassis of the appliance.
 - They are sharp, of course I haven't ever experienced this myself because I live in the US but apparently the rectangular beveled pins of the plug are very very painful to step on, especially since practically all of the plugs are of the right-angle style that can stick up when laying on the ground.
 - If you have a screwdriver then you can shove a Europlug in it, potentially damaging socket contacts

The plug that I think is the best (with conditions) is the type M plug (BS-546), most notably used in the UK before the late 1940s when they switched to type G, and currently used in countries like India and South Africa among others. Now the type M plug I believe is still made in unshrouded pin and unfused versions, and I believe the sockets are also made in unshuttered versions. But shrouded, fused, and shuttered versions exist, which maintain equal safety to type G with the following benefits:
 - Ground prong is too wide to fit in live/neutral parts of the socket
 - Rounded ends probably greatly reduce the pain associated with stepping on them
 - Completely incompatible with most cruddy international multi-fit sockets (a good thing, really)
 - Can handle up to 15A instead of just 13 (not as important for existing applications of course)
 - You can't shove other country's improper plugs inside them with just a screwdriver

Now obviously the UK switched away from this plug for a reason, because their ring mains could deliver up to 32A, and that required every plug to have a fuse. This could have been done with type M, but older plugs and sockets would still exist that remained unfused, unshrouded, and unshuttered. They made the right move in my mind. And all of the drawbacks of the current type G are really just nitpicking. But my viewpoint stands, type M is probably the best, assuming the sockets are shuttered and the plugs are fused and shrouded.

Also on a side note: Australian (type I) outlets are based on blades, like US plugs. New type I plugs have shrouded pins, proving that it is possible to make shrouded pins on a blade-style plug. Why doesn't the US do this? 120V can still really hurt you.

What do you think?

Aussie sockets have switches, so there isn't arcing! I never understood why the USA didn't do that.

@Baked bagel 11
Almost all of our appliances have switches on the actual appliance, that we would typically turn off before unplugging. I think Technology Connections made a video arguing both sides of the topic of switched outlets, I don't really know where I stand on that issue. In my mind, switches gotta be somewhere, doesn't matter where you put them. Switched outlets do exist in the US and are cool, but not often. Oh well.

Well, "simple appliances" (that is how the standard here used to call such devices; like hot melt glue guns, cloth irons,...) don't need switches, they can be controlled just by unplugging them. If done too frequently in one socket, you wear it down at it loses connection sooner or later regardless what mains socket standard we are talking about. Having a switched socket prevents that from happening, but on the other hand the switch itself presents more wear prone parts of the installation.

And for the British mains socket/plugs: Weren't they originally designed to work in 25A or even 32A circuits? So their design needed to anticipate at least 32A upstream fusing. Therefore the need for such beefy construction and the secondary fuse within the plug, as the 32A fuse won't be able to protect the cable from the plug to the device (or the cable would have to be too thick to be practical).

Most (practically all not based on the British one) other standards limit the maximum fuse (breaker) rating for domestic circuits to just 10..16A, so the branch fusing becomes sufficient for practical cable sizes without any need for any in-plug fuse. And allows the sockets/plugs to be way more compact.

So when judging safety, I won't agree the with the statement of the British system to be "the safest", when relying no moron would replace the blown fuse in the plug for a nail or so. With standards from everywhere else the cable is just protected by the breaker in the panel.
Yes, at the time of its origin, the power limitation the other systems impose semed as impractical, at that time the need was to quickly reduce the use of local coal burning stoves and convince people to use electric heaters instead, therefore the need for the rather high power sockets in homes.
Other parts of the world did notface that problem, so did not need such high power capable mains socket system, so went for simpler and more compact solutions.

But if you look on any modern wiring, there use to be way more branches than would be needed if the power rating would be the limit. So no need for 25 or 32A capable socket anymore, then the British standard (of such oversized sockets/plugs) becomes more of a historical relic than anything useful. But as with any other such standard, once set, extremely hard to change, even when others may be way better suited for present day needs.

I think our Swiss type J is the best. Bescause it’s small, safe and in some cases the 230V plug (T12 or T23) can be plugged into a 230/400V socket (T15 AND T25)

https://en.wikipedia.org/wiki/SN_441011

@Medved
While yes these sockets were installed on 32A circuits, each socket itself was only rated to carry 13A maximum (the largest fuse that could be safely installed in a plug).

And yes I am making a lot of assumptions here. Assuming the plug has a fuse, assuming the pins are shrouded, assuming the socket is shuttered, etc. Assuming all of those conditions are true, this system is safe. But there is no way to ensure that, which is admittedly a problem.

@Michael
Type J seems relatively similar to others like H, K, N, and O. Often no safety shutters or fuses in the plugs. Which is usually not as much of a problem since the maximum circuit size is 23A (but more often just 16A), but the conductors in the cords seem to not reflect that, just like the US. At least they have shrouded pins, that is definitely a good thing.

Interesting note on three phase compatibility, I did not know that. I can see how that can be useful.