Perhaps surprisingly, on several time scales in the last 365 days, the answer is STEEM.
Over Steem's lifetime, however, BTC edges out STEEM with 750 up days for BTC (blue and yellow in tthe graphs below, ~56%) compared to 661 for STEEM (blue and green in the graphs below, ~49%)
Also interesting to me, over Steem's entire lifetime, the two tokens don't move together as much as we might have expected. They have moved together (both up or both down, blue and orange in the graphs below) on about 66 1/2 percent of days.
*Numbers and percentages in both images are based on historical closing prices from coinmarketcap.com.
This is not meant to be investment advice. I just think it's an interesting comparison to satisfy my idle curiosity.