I would argue Tunisia. The disaster at Stalingrad still cost the Soviets dearly and more importantly delayed the 1942 Winter Counter-Offensive, allowing the rest of the German front to retreat in relatively good order and then severely blunt the Soviets at Karkhov. More importantly, the Germans cut their losses at Stalingrad, realizing that is was better to sacrifice an army than an army group. Meanwhile, Tunisia basically ended up with the Germans gift wrapping close to 250,000 prisoners, in large part because they reinforced a losing situation. You can also look at the end games, the 1942 winter campaign ended in the Spring of 1943 with both the Germans and Soviets in relatively the same position as they were in the summer of 1942. Meanwhile, Tunisia ended with the complete loss of North Africa, the destruction of the combat capability of the Italian army and the first real opening for a continental invasion.