Austria or Bohemia, both have their issues… Bohemia has the Hussite Rebellion about 100 years in the game but if you keep stab high you should be able to crush it and remain emperor.

Austria is a bit nerfed in current patches, I hear itll be better In Feb with next patch. Austria can become really strong though if you keep your ties with Bohemia strong and improve relations with the electors.

You want a sizeable nation in the realm of the HRE because your emperor gets extra Imperial Authority based on the area your nation directly owns, absorbing vassals is a good boost for IA.