but the whole point of the question was if you had to pick only one
Once upon a time the Sturmtiger did poor AoE and nobody liked it. Once upon a time before that it's shell blew up on literally every world feature, barrel, fence post, and geometry object making using it impossible.
If you see recon plane and you don't move your units, you deserve to lose squads due to B-4.
Sounds similar.
Or if you here a sound of B-4 and you don't move your units, you deserve to lose them
Hearing a B4 shot is pretty hard of over the din of battle, and it's impossible to predict were a B4 shot will land due to scatter and the fact it has 250 range. You also don't need a recon plane to use the B4.
There is a world of difference from an B4 1 shotting a PIV on the other side of the map and a Sturmtiger driving up to a 60 range unit that can scout for itself and killing it. If you let a Sturmtiger drive over an open field at you and kill you then just lol.
As Lemon said before; the go to counter to the Sturmtiger is AT guns, since the Sturmtiger doesn't have enough armor to reliably bounce shots and it can't kill AT guns (the gun itself) anymore.