It all depends on what you define as "best scorer." Could he be the leading scorer? Sure. As you pointed out, he only finished .8 ppg behind Pierce in 2008-09, meaning had a few rim outs and here and there gone different ways, he might've been the leading scorer.
However, I do question whether he could be the "best scorer" if we define "best scorer" as the guy you go to in the fourth quarter. KG is an insanely good player, but for all his skills, his weakest might be creating his own offense. Most of his baskets seem to come on jumpers and post-ups that are more a credit to his height and athleticism than quality of his moves.
Plus, I think clearly Paul Pierce was the go to guy in the fourth quarter, and Ray was arguably the second option.
Of course this begs the question, had PP and Ray been inferior scorers, could the C's have won the title? The answer is most likely no. However, you could reciprocally ask the question, had they been replaced with inferior scorers, but even better defenders, could they have won? Maybe. But I still have my doubts.
Overall, I think Garnett is the best player on the team. But I don't totally trust him as a go-to scorer in the 4th quarter. There's nothing wrong with that though. Bill Russell wasn't that guy either, and he was still clearly the best player on the Celtic Dynasty teams of the '50s and '60s.