Optical S/PDIF is the only reliable way to get high quality sound from a PC.

If you are using a PC for music or movies, you should avoid taking the audio from an HDMI, USB or coaxial output.  The reason for this is that PC's tend to create ground loops when connected to other pieces of equipment.   This is readily audible with analog connections, but insipid in digital.

The issue with digital is that you'll get a ground loop introduced into the signal which can cause very high amounts of jitter and you'll be listening to just OK sound which you won't notice until you get something better.

There are some exceptions though.  Unfortunately magazines like Stereophile won't test these options.  They deliberately use test in a way that negates the benefits, so you can't tell from a review if your DAC has good isolation or not.

Using an optical connection between your PC and audio system guarantees that you won't be listening to a DAC under the worst possible conditions.

There have been some that argue that coaxial S/PDIF connections are better, but those arguments were made assuming dedicated CD transports and DAC's which have far less ground loop and noise issues than PC's.

So, until something better and reliable comes along, I'd suggest to always use optical.