Depends on if you're gil farming or CP farming. For gil, I like the Adamantortoises are probably best as you can continually repeat Mission 63 (which automatically nets you a 60k gold nugget) and the two other Adamantortoises which are nearby. For CP, the Adamantoise in Eden is probably fastest because of the nearby save point.

I personally prefer the Adamantortoise location in the Steppe. Other people prefer Eden.