The Yonko, specifically due to Shanks and Blackbeard. In the grand scheme of the series, Sakazuki is more important than Kaido and BM though.
The least plot relevant of them all is Kizaru, by quite some distance (so far at least). But the Admirals in general suffer due to being subordinates (of a subordinate, really, given Sakazuki isn't in charge of the World Gov).
In terms of plot relevancy I'd say it goes
Blackbeard/Shanks
Whitebeard/Sakazuki
Kaido/Big Mom/Kuzan
Fuji (and presumably Ryo, but that's just a guess)
Kizaru
And I suppose I should mention Sengoku as well, who's probably in Kizaru levels of relevancy.