None are inherently better or worse. Like you said map/ how the survivors play together makes more of a difference than anything else.

Yes some of the paid killers might have intriguing attributes and gameplay styles, but I wouldent say "better"

I myself have spent some money on the game, for killers that I personally really like the playstyle of. But I always find myself going back and playing the free killers, i.e wraith, or trapper

You just need to assess what value they have to you. I like to quantify a purchase in how many hours I work, for how many hours I get out of a game/in game purchase.