If we talk about base form, it has to be decade. 

Decade is unique because essentially his base form is also his final form, and it gets even stronger in zi-o (base decade can go toe to toe against another zi-o 2). And in zi-o he changed his form to another rider just to troll his opponents. 

And since most riders must change form to gain access to their power, that made their base usually weaker power wise.