For me it's hard to say. In terms of losses, Paulus and Varro (Cannae) made the worst mistakes, along with Maximus and Caepio (Arausio). As both lost huge numbers of men. Each battle, the equivalent of two army groups.
However, the slaughter of Quintus Varus's three legions (20,000 men), had huge consequences. It came in a time when Rome was establishing itself as the worlds greatest empire. This involved expansion of territory on that already held. Including a greater incursion and presence in Germania.
During these times, the army was made more professional, and loyal to the emperor, not governors. With a sum of 28 legions standing. Varus's loss made a huge gap in these plans, as well as border security. 10% of romes army was lost, which made a huge impact.
This was all due to Varus's incomptence, in leading his men into a trap, in an area that was obviously more to his enemy's advantage. As well, the closed forest did not give room for his men to coordinate counter attacks or defences. Only get cut down. Because of that loss, the Romans had to wait years before they had sufficient forces, to make another incursion, and try to find the slaughtered remains of their army.