Short answer: Klein.
Long answer: Setsuna requires more, effort, investment, dedication to reach what Klein has naturally with just his prf.
Setsuna has been popular BB+ user because she can consistently quad with a brave bow.
However quadding alone doesn't guarantee kills, Setsuna will very often "0x4" on any unit that isn't a flier or a mage. She doesn't kill those units with attacks, she kill them by spamming guaranteed Luna that occours every quad cycle. Her high speed guarantees she'll quad even against very fast units and luna will allow her to bypass most defenses.

The issue is that Spd doesn't exactly translate into damage, it allows and prevents doubles but ever since breaker, fighter, sweeper skills/weapons, dancers, brash seal and most importantly cheerleading teams exist Spd has become less and less relevant for ranged Player phase units. Attack points on a brave bow are twice as effective and if you have enough speed or a follow-up skill they're four times as valuable.

Klein even before his prf was better as well as other archers of his category: he was still fast enough to kill mages/fliers along with fast infantry units by just using regular brave bow attacks while slightly slower units were instead in his quad range like def mages (Robins, etc) or older armored (Hector, W!Lyssa etc).
His prf weapon secures his role as the current best 4* BB user, as he'll easily outperform any other archer with merges.
You only can do better with +10 seasonals like +Atk Bridelia or similar offensive spread of new gen units (or Bold Jacob), but then you're still losing out on the +Eff of his prf weapon.