I don't know about "best," but a couple of useful sites with sys 6 apps are:

home.earthlink.net/~gamba2

and the System 6 Heaven site:

http://www.euronet.nl/users/mvdk/system_6_heaven.html

Gamba's site also has links to many other sites. Some of the links are dead now, but a great many are still live.

There's also Le Grenier du Mac (the Mac attic), which has a mix of 6 and 7 apps:

http://www.grenier-du-mac.net/

These will keep you going a good long time. There are other great ones, too, and I just know that others on this Forum will jump in and let you know about their faves shortly.