I voted for Spain for a couple of reasons. 1st and foremost they could have than taken out Gibralter securing the Med. Secondly the Uboats would have had more docks. 3rd More airfields would have left the shipping lanes in the Atlantic far more vulnerable to attack. You add another 1000 miles westward into the Atlantic to scout for subs or just outright attack them by air yourself.
Had Spain joined and done this Malta would have fallen from no supply. Troops in Africa I believe could have pushed up and threatened the Caucus from the south (hell of a supply line on that one though). England would have starved and I believe it would have been invaded. If you don’t have the oil/parts you won’t be able to send fighters up to defend.
Operation Barbarossa would have been post poned for a year while Sea Lion was fought (and won) by the Germans. With no force in the west (And all the resources of england to help) to defend against Hitler could have moved everything he had against Russia in 42 instead of 41. Which would have avoided the 41 blizzard.
It would have been a much different war, much more favorable to the Germans.
The biggest mistake the Germans made was not insisting that the Japs attack Russia. That one thing could have won them the war. The Germans would have won the battle of Stalingrad if they had and pushed onto the Caucus.