Riot. At first because I see job listings from them all the time but I was comparing https://ir.mara.com/news-events/press-releases/detail/1313/marathon-digital-holdings-reports-first-quarter-2023-results and https://www.riotplatforms.com/investors/news-events/press-releases/detail/152/riot-platforms-reports-first-quarter-2023-financial.

They are both good I think but Riot is bringing in more cash with less hardware which to my idiot brain means they are a little more optimal about their operational costs. However if total Bitcoin produced is your reason for investing then maybe Mara instead.