Gotta be blunt: for raw Slayer XP in OSRS, Duradel and Konar are the heavy hitters.
Duradel hands out higher-tier tasks reliably, which means you spend more time on beasts that give proper Slayer XP and less time on filler grinds. If you pair his tasks with a Slayer helmet, decent gear, and smart usage of cannon where allowed, you can push your Slayer XP per hour way higher than low- and mid-tier masters. Konar is a special case — she hands out Wilderness and area-specific assignments that can be insanely fast XP if you can handle the risk and mechanics. Her tasks often let you head to compact spots with dense spawns, and if you’re confident in PvP-avoidance or have a safe setup, Konar can beat Duradel for raw hourly numbers.
That said, a lot depends on what tasks you actually get: Gargoyles, Dark Beasts, Abyssal
Demons, and Waterfiends are the kinds that spike your XP/hr. If you’re still working on gear or player-safety, Nieve (or Steve on reworks) is a solid mid-tier option giving consistent, efficient tasks. For absolute beginners, Vannaka and Mazchna are fine, but they cap your XP potential. My personal takeaway: level up to the highest master you can reach and learn how to manipulate or block bad tasks — your XP will thank you.