Which of David's mighty men killed a lion in a pit on a snowy day?

Answer: Benaiah son of Jehoiada

Benaiah also struck down two champions of Moab and a huge Egyptian with the Egyptian's own spear. David put him over his guard.

“And Benaiah son of Jehoiada was a man of valor from Kabzeel, a man of many exploits. He struck down two champions of Moab, and on a snowy day he went down into a pit and killed a lion.”— 2 Samuel 23:20 (BSB)