>>30276419Merciless would be a great ability if Toxapex was more offense orientated, but it has very low attacking stats. The ability itself is also null on Steel and other Poison types.
Regenerator is better because it's a free heal that goes in tandem with Toxapex's natural absurdly high defenses on top of Recover. Toxapex can take a bunch of damage, use Recover, then switch out, and thanks to Regenerator, come right back in with full HP the next turn. In short, Regen fits Toxapex's role as a needle fortress better than Merciless, though Merciless would be insane on any other poison type.
Lurk more before asking questions. 
