The answer from most people will always be summoner, and it’s very difficult to convince people otherwise, but I disagree.
For a pretty large fraction of the game, the DPS of a summoner outpreforms any other class. A Spinal Tap, Obsidian Armor and Vampire Frogs for the Wall - or Hallowed Armor, a Sanguine Staff and a Durendal for Plantera are some of the strongest builds you can get at their stage of the game.
Early game, Flinxes are very powerful, and the combined DPS of them and a whip is pretty high for beginner game. Then Obsidian Armor - even after its nerf, allows Summoner to work on par with other classes for Bee and Skeletron and actually do better for the Wall of Flesh than any other class thanks to the Spinal Tap. This trend continues the entire game, with Summoners having options that compete well with or outpreform other class options: Blade, Sanguine, Desert Tiger (after its buff, it can kill expert Empress in under a minute with the right build!!), Raven, Xeno, Stardust Dragon, Terraprisma.
This is balanced by being squishy. You are with low defense (which doesn’t matter that much actually, defense is usually not that much of a difference unless you stack up a ton of it) and are forced to be next to the enemy. But for putting yourself in such high risk, you are rewarded with phenominal DPS that other classes can’t even begin to compete with.
If anyone doesn’t believe me, try it. I used to be part of the summoner bad circlejerk until I actually did a run with the class and found my preformance to be impressive for the majority of the game.
As for which class is the worst, that’s melee in my opinion. Melee struggles for much of the game, lacking good options for core bosses such as the Eye or Brain of Cthulhu, (especially) the Wall of Flesh, or Plantera. Until post-Plantera, most of the weapons can’t even compete with those of other classes, and only a few weapons (ex. Shadowflame Knives) are even worth using for bosses. Melee makes a 180 turn immediately after plantera, but it’s so weak for so much of the game that I can’t call it strong.
and don’t you dare say anything about zenith - zenith is a final, endgame weapon and its existance changes nothing about melee’s preformance for the majority of the game.