Well, that depends. Do you need to be independent when you become king? A duke in the HRE or a catholic count/duke in the ERE could spend all their resources preparing to participate in the first, and then have the protections of their lieges (the quality of what the ERE can offer greatly varies game by game). You could also be any powerful duke, in France and then get your independence when you win Jerusalem. Toulouse seems an obvious choice. If you're willing to be part of the HRE for some time, also the duchess of Tuscany.
Otherwise, maybe Dulkja? There are other powerful dukes of counts with potential, but the issue is the distance from Jerusalem means keeping it alive is very hard once you get it and before holy orders and conquest penalties disappear.
I'm playing as the independent count of house Spartenos in Sicily, swore alligance to the duke of apulla, converted to catholicism, then swore allegiance to the ERE when he went belly up against an islamic invasion and took most of it back personally. I never win the crusade, but I hold Galilee, Tripoli and some of Damascus since the Fatimids had a major rebellion. I plan to be KoJ before 1150.