I've never understood this argument. If you want to say that Paul Pierce is the most versatile offensive player the Celtics have had, or even the most skilled, I'll listen to your argument.
But the best? Even if you confine the argument to just scoring, Larry Bird scored more points, and converted his shots at a higher percentage. Of course, if you factor in other parts of the offensive game, like offensive rebounding and passing, Larry blows Pierce out of the water.
So, in summary: More points + higher FG% / eFG% + more assists + more offensive rebounds = better offensive player
I take that quote as a direct slap at Larry, since it seems like Parish has him ranked behind both Pierce and Hondo. It shouldn't come as a huge surprise; Larry wasn't especially well-liked by a lot of his teammates off the court, which explains some of the negative statements by Max and Chief.
Also, even forgiving Parish the indulgence of grouping himself with the best scorers in Celtics history, how the heck did Don Nelson make the cut?!