This is very tough and will remain subjective.
Kudos to the Luftwaffe and USSR for Russia wnters
Kudos to USAAF Aleutian Ground crews for extreme wind/rain and cold maintaining 'everything'
Kudos to ANZAC, RAF, USAAF and Luftwaffe for maintaining their ships in the desert
Kudos to the USAAF ground crews for solving amd making field changes to all aircraft that the designers hadn't quite 'fixed' before shipping
Kudos to RAF and USAAF crews working round the clock night after night to get their beasts up for another Maximum Effort in that bloody English goo and foul weather November through March (my Texas born father, when asked about English weather exagerated somewhat - "It has two seasons, Winter and the 4th of July)
Kudos to Luftwaffe crews for surviving strafing attacks, moving from one location to another and setting up again - only to move again.
Kudos to the Pacific crews of all nations and services for the 1941-1943 conditions and primitive facilities like Guadalcanal.
Someone is going to have to lay out some serious boundary conditions to select one.
Having said this -Just for dealing with a complete range of conditions, from Artic to Desert to Jungles to Sea to Hurricanes, Blizzards, Ice and Sandstorms - and bringing Southern Engineering with them - I'll be a 'homer' and go US.
Regards,
Bill