If Sargeras gets to join the field, then the Old Gods do too. One on one, Old Gods seem stronger than Titans pound for pound. Dunno how a single Old God would stack up against the Dark Titan, but 3+ would take him.
Without either of them, purely in terms of their forces, probably the Burning Legion. Without the Old Gods to work their corruptive mojo and subvert the enemy forces, I think the Legion would eventually overcome and burn out the Old God forces. Put even one Old God into the mix though and he'd have half the enemy on his side before long.