Echoing Ar7's sentiments, Von Holstein, please. Yes, the notion of 'liberating' Constantinople, 'reuniting' the Greeks with the True Faith and restoring a catholic Eastern Roman Empire is a nice romantic gesture, but it is based on such flawed reasoning that, frankly, I think the Order would have more success if they put a mute, amputee macaque monkey in charge.
Let's see... The Fourth Crusade utterly failed in establishing a viable Latin state base on Constantinople, and it faced far weaker opposition (I mean, strife-ridden Byzantium versus warmongering Ottomans? Not exactly a favorable comparison).
More importantly, even if the Order could somehow organize a crusade that would be strong enough to take Constantinople AND kick around the Turks, could find the fleet to get that Crusade from the Baltic to the Bosporus, could get enough money to maintain those forces for the decade or longer it would take to put the plan into motion... Even if all that were not just possible, but even feasible, it would still overlook a tiny detail: all of the Order's neighbors hate them with a passion, have attacked them at all opportunities and have absolutely no reason to stop doing so now (or, more accurately, there is only one reason: it's called the Teuton army).
If von Richtenberg can denude his frontiers of all troops, ship them off to Byzantium and start traipsing all over the Balkans for 10-15 years without his neighbors (catholic, orthodox and/or simply opportunistic) declaring war en masse... well, to wax anachronistically, then it is time for Satan to go to work on a Zamboni.
Von Holstein, because sanity is just too valuable!