I went back and forth on this clicking each duo around 4 times, but ended up landing on the young two. I don't think Hughes or Makar will ever have the impact Keith had in terms of stopping the best forwards in the league as well as he did. It really came down to how dominant these defensemen are against their era's. Makar has been the best defenseman in the league for a few years now and he is still so young. Hughes has also shown dominance from a young age. Keith was a dominant force everywhere, but I wouldn't say he separated himself from the pack by a whole lot or for too long of a time
It all comes down to how long they can sustain being the top dogs at their positions. Erik Karlsson was the best defenseman in the league for at-least two years of his career and I can see Makar/Hughes having a longer stretch.