What is the best mod for a Sony SCD-1

For one get a Marigo Mat either regular or signature. Detailed but smooth.

having had both the kern mods..almost the full mod..and the vse mods the vse mods ..imo..give the better sonics by far..others may believe otherwise.

That's simple, take a look at the following review...
"Tube Research Labs" MOD is one of the best, most affordable, and effective approaches you can get.

http://www.6moons.com/audioreviews/stephstuffers/stuffers.html