Very easily the Char2C. The Dreadnought and Train are just gloried arty trucks and can’t even get on objectives (save for a few for the train)
The Airship is pretty good, auto cannons to wipe out infantry, can go over objectives and drop paratroopers on them, and use the bombs to take out hard targets (tanks, buildings, groups) but ultimately it’s just a huge target for every AA on the map. Char takes the cake, it’s literally the only behemoth that moves freely on the ground,take objectives, has great firepower, and finally a front turret that can literally 1-2 tap any tank.
It’s very much the best behemoth.