Best for mostSusan Bernofsky · 2014
Susan Bernofsky's 2014 translation is the one I'd start with — fluid, contemporary, and careful with Kafka's precise, deadpan German (she renders the creature as 'some sort of monstrous insect,' keeping the ambiguity Kafka intended). This is where I'd start.
Best for studentsStanley Corngold · 1972
Stanley Corngold's translation — 'a monstrous vermin' — is the scholar's standard, prized for fidelity to Kafka's cadence, and his Norton/Bantam editions are rich in commentary. The one for study, and very cheap.
SharpestMichael Hofmann · 2007
Michael Hofmann's Penguin translation is the most aggressive and unsettling — he goes as far as 'a monstrous cockroach,' which some find too interpretive, since Kafka deliberately never names the creature. Vivid; not the most neutral.