>>30477736Merciless is better in-game since it speeds up the fight, stalling opponents is boring and time consuming during your playthrough.
For post-game content like Battle Tree/Battle Royal and for online PvP, Regenerator is undoubtedly the superior ability.
I don't think any other Regenerator pokemon has as even defenses as Toxapex has. You can switch in on an attack and as long as you took less than 1/3 switching back out puts you back at full health effectively nullifying your opponent's move.
