IMHO the popular is the best, but also the most difficult one, as the Germans are quite eager to have Democrazy.
Baring that I would go for the conservative one, great for your prestige/economy, unified Germany is just big... Great manpower to fuel those factories (wars being fueled by my colonies...) and territorial claims on France and Denmark.
Beware of a multi-front war (i.e. against Russia/France/Austria) and that BB-value that will get you into then eventually... After the unification you should have a very large army to deter those pesky Russians/French, keep the freindship with Austria (or get one with one of the other two). You can consentrate on the army early on, do it. Skip the Navy for later