I think the answer the thread starter may be looking for is the upwind engine.
During the takeoff roll, before the aircraft is flying, there is really no P-factor to talk of (assuming the propeller disc plane of rotation is perpendicular to the runway), thus the thrust line will be more or less through the centre of the propeller disc. Factors such as slipstream may still be a factor, though.
However, since "critical engine" is define as the engine whose failure would result in the most adverse effects on the aircraft's handling and performance, one could argue that the upwind engine is critical. Obviously the vertical fin would, in a crosswind, add to the yawing effect of the downwind engine, while it would counteract the yawing from the upwind engine, if the downwind engine was to fail.
Vmcg, as certified and described, does not allow for a crosswind. However, the speed at which the aircraft is directionally controllable on the ground in case of an engine failure, will in fact vary with crosswind component.