Duncan. KG can defend SFs and whatnot (I'll say slightly above average at that), but Timmy can defend centers and PFs as good/better than anyone in his era. KG could never defend centers, and was decent at defending PFs.
KG isn't really a great man defender. He's great at coming in on the weak side for help defense. Sort of like Serge Ibaka, only KG is a better man defender than Ibaka. Timmy can stop bigs by himself, and is a great help defender.
Tim Duncan easily. KG being able to defend a SF is more of a luxury than a neccessity. It does not put him above Duncan simply because he can do something no sane coach would have one of his 7 footers do very often.
Kevin Garnett is a better defensive leader and a more versatile defender. Kevin Garnett's defensive impact > Tim Duncan's defensive impact.
A past his peak Kevin Garnett instantly turned the Boston Celtics into the best defensive team in the league.
This, im taking Garnett because he was better at protecting the paint. Duncan was beter defending around the rim but I think that's got more to do with Garnett in Minny having to be ALL over the floor defensively for the team