Funny this thread should come up - I was just wondering this the other day. I’ve mostly called it ESB, probably because that seems to be the more common abbreviation, but realise this is inconsistent with the other films. For example, people tend to include the “T” in “TPM”, “TFA”, “TLJ” and “TROS”.
If searching on Google for things related to Empire, “ESB” is far more lucrative.
I think the reason why TPM, TFA, TLJ, and TRoS caught on where TESB did not is because the first four only have two words if you exclude grammatical particles, but the Empire Strikes Back has three. Same thing with A New Hope, because if you can exclude “the” you should be able to exclude “a”.