What kind of biased vote poll is this?
Doesn't included all assassins and only includes the one that the OP thinks is fine?
I only listed assassins not commonly complained about on the boards frontpage, and are generally considered to be assassins.
What?
Zed is an assassin with the most interactive assassination sequence. Such thing is considered "healthy" by Riot. He just happens to have a great laning phase, which most people complain that assassins should not have. But his teamfight potential is so pathetic when the player is at equal skill level as his opponents, that split-pushing/dueling became the main role of Zed, otherwise he won't have anything worthy to bring to a team.
Other assassins such as Talon are balanced by a weaker start to the game, and compensated for more reliability to assassination sequence. People doesn't like their kill potential but can't prevent themselves from feeding them early anyway, when they are at their weakest.
Fizz is a very weak laner that gets complained about routinely by Boards because they don't know how to play against him in lane. Every time Fizz kills someone in lane without jungler support I face palm...
Ekko is an AP assassin that has zero reliability whatsoever unless the enemy is dumb. The best use of his kit is as a stun initiator. No wonder tank Ekko is so popular, because his kit really doesn't deliver damage. But he has his signage ult and of course, people complain about that.
And all assassins do not do well against a meaningful support. Assassins that are most successful always have a dive-buddy with them to force supports to blow CC, otherwise they cannot perform their intended role and is most likely weak/useless until their main rotation comes up.
The counterplay to assassins is always awareness and teamwork. Dueling against assassins usually don't work out, unless its laning phase when really, assassins and other laners are on equal footing.
There really isn't a most "healthy" or "balanced" assassin that "the boards haven't complained about".