Which courier do you prefer?

So far more than two thirds of the opinion favor USPS.

Good for them.

But there may be lot of differences depending on region.

I myself had not much problem with all three of them until recent delay by Fedex.

And only USPS delivers to PO BOX, I guess.
Pack especially well for UPS and FedEx and then double it. They can be very rough with packages.

USPS has been very reliable to me as both a buyer and seller. I have an excellent post office with their current staff, so that always helps.

Fed Ex is very reliable but often slower than expected. Never trust their tracking estimates (in my experience); it takes 4-5 days to move through Connecticut for some reason, but they will clear the entire midwest in a day or 2 (I realize the latter may be air travel, but Connecticut can be traveled through in a couple of hours on land and a stop at a distribution center shouldn't take multiple days). They also have a nice staff at the center near me.

UPS I've mainly shipped through for returns to major companies and it seems to work well. Their staff is nice at the center near me as well. They deliver without issue here, but I've rarely had interactions with receiving things from them outside of major companies.
Luckily I have a safe neighborhood, so packages are never stolen and if something is delivered to the wrong address the neighbor will hand it to me in person.

DHL express service is really fast.

Psavne Acme 805 tube shipped on this Tuesday in China is going to be delivered at my home in Washington State on Thursday.

DHL should be fast . Last year I got a quote from them for a 30 lb package from Minneapolis to Berlin .  They seemed to think $2,200 was
a good deal .