GMC 2½
definitely
I have read German descriptions from Winter 43/44, full of complains how Soviet troops pushed through mud in LL trucks while their own trucks, Opels etc, got struckt even on the roads, not that there was much difference between cross-country and road in Ukraina in that time during thaw season. Have not time to check the British vehicles but generally US trucks tended to have more powerful engines than European trucks.
What's there to like about Opel Blitz??
It' a civilian truck designed to quality roads, so inherently not suited for the Russian terrain (mud, rasputitza, snow etc.),
US truck of just about any kind is way superior to what other countries produced.
Gaz AAA (not the 'anti aircraft artillery', but the Russian designation) 4x6 and ZiS-5 2x4 were the most common. Decent trucks, but US stuff is better.
According to what I've read the Opel Blitz was very popular with the Germans, and it did well. Sure it didn't have as many horsepower, but it was a lighter truck as-well. The German heavy trucks were everybit as good as those the US built.