Go Intellect is probably the strongest program that is still available for Mac OS 9 (won't work in Classic). It is available from Yutopian.
Goliath was also strong in its day, but I don't think it is still for sale.
Nemesis, the first Go program for sale to the public was available for Mac OS 9.
Macintosh Go owes a debt to Anders Kierulf for making his Smart Go Board available to the public back in the 80's. It is the foundation of both Go Intellect and Explorer, as well as the first implementation of the Smart Go Format.
See also Macintosh Go for an overview ofsoftware for other macintosh versions.
The website for the British Go Association makes it clear that Explorer is the hands-down favorite among Macintosh Go programs (all games referenced by myself are available on the BGA site).
There can be little doubt that Explorer is the most skilled (with the possible exception Goliath -- I'm too poor to buy it), but as a diehard aesthete, I really loathe Explorer's look. It's so pre-Mac LC.
Lumpy seems to do the best job of presenting a pleasing arena while keeping the file size under 600MB (Goliath's demo alone is 2MB, and while the arena is gorgeous, play mechanics are awkward and the music is atrocious). The problem is that Lumpy's play is .. well, lumpy. I beat it consistently within a month of learning the rules of Go.
SecretMojo interjects: " Quickie Go is a clean interface to the GnuGo engine, works on Classic and 68k, and has other features as well."
What we need (and I think this applies to Go in general on the computer) is a well-made Go arena with modular AI. Does anyone know if such a thing is in the works? If not, why not?
For a while I was baffled at how Explorer doesn't count all territory around dead stones; this is explained in AncientChineseRulesAndPhilosophy. Some of my other beefs with Explorer include preferences that seem to reset automatically, awkward placement of numbers (leaving tiny trails), and a temperamental relationship to the Applications Menu (it may be my particular setup).