Constantinople is easily the most vital province, and the only province that you cannot realisticly achieve the 60% "lesser victory" without taking at some point. not only is it a vital chokepoint, but it also comes with a heavily upgraded and outfitted castle, so you need to either take it and use it, or take it and burn it to rob anyone else of it's advantages. but that's all fairly obvious.

other than that, rome is essential. not to hold of course, but to manage, and take and re-take frequently. esspetially if you're a catholic factiona and want the pope off your back whilst you stretch your legs. consequently, this makes tuscany and important province to keep as well.

aragon is a pretty good province to maintain, as it is easily defensable, and borders many other important provinces that can be harder to keep. also, provence for the same reasons, and because it makes a fine staging area for major western and northwestern assaults.

finally, the black sea is necesary to control if you wish to leave the northeast ungaurded while you push westward