Best Gallic tribe to play as?

[Sammy]

In order to unite Gaul

[Attackmack]

The one with a small village that rome never manages to conquer :D

[Kelldath]

It's actually one of the best spots. Start in Brittany, being mostly surrounded by the ocean, you have few neighbours to worry about at start, can choose which one to take down first.
Expand north east until you reach the empty lands. You'll be a large tribe, but half of your borders will not be threaten by anyone. This makes conquest of the rest of Gaul much simpler

[Kanabistro]

Form Belgia and then you can conquer the rest.
But belgea culture is super easy to unite.
It will give you a nice start for colonize the german and no enemy to worry from the nord.