The Shichibukai by far.
Their roles in the plot are way more varied (from villains to allies and even a crewmate), have been a explicitly addressed and well established group since day one (I mean, Jinbe was mentioned in East Blue), their crews not only are each very distinctive but one of them (the Donquixote Pirates) is arguably the best finished up group overall, their individual designs are pretty nice (and Kuma is more than likely the best in the whole series, it is virtually perfect in terms of concept and execution), their dynamics are rich as we have all kind of personalities and achievements (from winners to losers)...
The Yonko just feel flatter. I've never been a great fan of the Red Hair Pirates nor the Whitebeard Pirates, the Big Mom Pirates are my favorites in this group (so is Big Mom among the emperors alongside Teach), the Beast Pirates are disappointing and we need to know more of Blackbeard and his crew. Their individual designs are way under the Shichibukai, so are their concepts (Big Mom has the upper hand here), and kind of feels like their plots and roles are less imaginative and distinguished from each other's.
I guess that, at the end of the day, Oda just had more freedom to write the Shichibukai since they aren't chained to the "template" of great, strong pirate ruling an empire in New World. So it is easier for them to just be more interesting overall.