I think Sabo vs Akainu has a better shot.
From most likely to least likely:
1.Luffy vs Akainu - their clash is guaranteed to happen
2.Sabo vs Akainu - may or may not happen
3.Dragon vs Akainu - least likely, Sabo is from the same faction (so the rev vs Akainu angle works the same way) but has more points to his favour (new gen > old, Sabo carrying Ace's will literally and figuratively, Sabo's regret)