I'll say Coruscant is the most important planet in the Star Wars Universe.
Coruscant was the heart of the Old Republic, and later became the heart of the New Republic, as well. It was the headquarters and barracks of the Jedi Order, and the founding place of the Rebellion. It wasn't just of political value, either... All of the major trade routes lead to Coruscant, co-ordinates 0,0,0. Being the center of trade and politics, its a given that it is of military importance, as well. General Grevious and Grand Admiral Thrawn were alike in they realized striking Coruscant, even in a last effort gamble, would demoralize their enemy and possibly force a settlement.