Which tribe is easiest to defend?

I would say it would be gauls. They have a lot of good defencive troops. Although if we look generaly, the best would be a combination of roman pretorians, and teuton spearman for calvary

However, the Teutons' Paladins and the Gauls' Druids are very good defenders.

Don't forget about the trapper though. They do lose some value as the game goes on but for the first couple months just the idea of traps turns most people away from Gaul targets.