Unfortunately, so much depends on what Germany is doing. I’m loathe to land units in a territory (WEu, Nor, Kar, Arc) where a German stack is just going to obliterate them for the cost of a few inf, but sometimes this is your only option.
I’ve sort of found that landing in Arc leads to a longer game, where the Allies are more sort of reinforcing the Russians as opposed to actually striking out on their own against Germany. This can even be the case for Kar sometimes, too. So it’s really more of a question among WEu (uber aggressive), Nor (usually safe), and Kar/Arc (conservative/safe).
If Germany is putting serious pressure to deadzone certain territories, then I’d certainly go for the other one(s). Anywhere where the western Allies can start building up a stack is a good place. You don’t want to keep throwing 4inf 4arm into German stacks.
But, if you can choose, I’d say Kar is the place (or, bv extension, just marching through Nor into Kar). It allows for reinforcement into Russian territories and a march through Germany’s lines.
WEu can be quite a sucker punch. Once the UK holds it, they can maybe drop an IC to make use of their extra income, and once the US takes Seu, Germany is like 1-2 turns from dead. Not that I know this from experience, but just looking at the numbers Germany must have a ridiculously huge stack on Ger/EEu or Weu/Seu/Lon/EUS will overrun it very quickly.
It really depends on the G player whether WEu can be taken. Even among the most aggressive (who abandon WEu early), it can still be hard to make a strong landing that leads to UK holding WEu and US taking SEu. Against more so “turtle” players your best bet is to avoid the heavily defended WEu and just go Nor/Kar.
However, a key thing is to look at the board. If Russia is really straining to survive, maybe some reinforcements will get it out of a tough spot and ensure that Mos or even Cau doesn’t fall. But if the Allies have a sizeable lead, it’s only going to get larger: Make calculated moves through Nor/Kar/EEu and just slog your way into Ger.
Really, G shouldn’t be able to defend both Kar and WEu for more than a few turns, so once the pressure is off, you look at the board, choose north or throat, and go go go.
I completely agree with overbuilding the UK on transports (usually the Aus trn becomes the fifth), and with that and an efficient US, you should be making G sweat bullets.