No I meant German unification. For HRE unification Bohemia is also an optimal choice, but HRE record is close to optimal, I think. The limiting factor there is number of diplomats. Your only hope is to get lucky alliances (so you can drag maximal number of enemies into a war at the cost of a single diplomat), lucky annexation (when someone annexes someone, so that you can get both religious unity and nation release bonus at a cost of a single diplomat) and that random event that gives you extra diplomats.