depends on what you are using to generate it. each plugin or technique does it a bit differently with different results. Some use phase, some use level, some use phase and frequency etc... The goal when using something like this is to spread the stereo width wider without destroying the image. If it's needed, usually a little is good, a lot it bad. A lot of stereo enhancers will tend to collapse the middle.

They can also be used to tighten the stereo field. This can result in a punchier
mix.

Which of these plug ins or hardware produce the best results?

Bobs Katz hardware unit is supposed to be the best.

voxengo ms plug is good

Also harbals air feature adds a great little touch.

T-racks compressor has good stereo enhancement.

DUDE007 wrote: Which of these plug ins or hardware produce the best results?

There are several techniques (normally combinations of steps) that may or may not be *driven by* plugins or M/S matrixes. Which method works best is entirely up to the mix.

And I'd argue that most of that should be well settled before the mastering phase...