In the UK, Coastal Command (whatever they flew). Hated by the rest of the (bomber obsessed) RAF, starved of resources.
Long, very often boring flights, in often horrible weather over the sea (with very little chance of survival if anything went wrong), intermixed with sheer terror.
More often in obsolete planes.
Just 2 squadrons in Iceland (VLR Liberators, Mk 1s) stood before outright victory for the U-Boats in early '43.
Harris hated them, stated that they 'were an obstacle to victory', Portal tolerated them because of politics (though basically he agreed with Harris and begrudged and whinged about any resources spent on them, in between the time he spent on sabotaging a LR day fighter, night fighter, support for bombers and CAS that is).
Forgotten now, but their contribution to the (must win) Battle of the Atlantic was incalculable. In many ways, the best of the best.