I would say Lily's corpse, but I've always imagined it as something a little più in depth then that, because thats what everyone thinks it would be. I personally think that it would be himself as a death eater because in his mind, I think that represents all the bad he has ever done, and in his mind, himself being a death eater ultimetly led to Lily's death
I think it dependes on wich period of his life he is in. When he is in his 6-7th anno in might be lily with James. Then when Lily and James go into hiding it would be Lily Dead. I think It would change multiple times through out his lfe but when he is about to die I think it would be Harry Dieing and Voldeermort livind because he would think he had failed Lily, and Dumbledor.
Maybe when he was younger it would have been his abusive father.
Now i think it would have been some sort of scene unfolding before his eyes. I would imagine Harry's death (because he would have failed Lily) o even himself holding up his wand and killing Lily instead of Voldemort (because in his mind, he is the one responsible)
Honestly I think they'd finally get to see what a Boggart really looked like if one was to appear to Snape. Imo his worst fear has already come true ie Lily dying and him being responsible for it, so since he's already faced that, not much else would affect him. He didn't care about anything else not even his own life. That's why Dumbledore knew he could trust Severus.