The least important game in the series?

[Hollycat]

You'd discount the origin of the switchable job system? 3's contributions to the series are far more important than anything offered by 6, which was basically 4 with switchable characters. The concept of equipable summons lasted another two iterations before being scrapped (and was even introduced in 3, if you're counting the fact that all magic could be equipped and then unequipped).

If you want to find the least important, you're definitely looking at 6.

Honestly, I was struggling to think of things for 6. But I never finished 3 so that really wasn't fair.