This is a very difficult question to answer with complete certainty seeing that it is a hypothetical. Stalin made lots of people, even high ranking official, disappear, so while it is unlikely that he would hesitate to do so, this is sort of a mind-reading exercise across time. If I were to pick names of people that Stalin would have had a hard time killing, probably Zhukov (especially after WW2) and Beria come to mind. Zhukov was famous around the world and was useful during the war. Beria, being head of intelligence, had a lot of power, and after Stalin died, even claimed to have done him in. Keep in mind, though, that Beria's predecessor, Yezhov, was killed after a couple of rounds of machinations (reassigning him to a different post and so on), so it is not impossible to imagine the same fate for Beria if Stalin felt the need to kill him.