To me it'll definitely be bitcoin.
I expect a recovery of BTC prices on a huge scale within a year, and the bull market should come by at around 2020 since that's when the halving occurs, and the most hype is built up for BTC trading. It's not necessarily go as high as the last bull market, but certainly you can expect at least 200-300% gains from BTC recovering from the current value to its previous ATH alone, and that's not taking into account that the last ATH is most likely going to be eclipsed during the next bull market.
Berkshire could continue its 10-20% gains per year, which is great. But BTC is still in its early adoption phase, and as such, will produce much better returns. Buffett will regret not getting into BTC early.