If I took the approach of rating an enemy based on visual creativity, unique mechanics, how fun they are to fight, and then if their drops are worth it...
I think the worst boss would vary a lot. There's bosses for each category, which are amazing in others. Duke Fishron for example is the perfect boss, it's full of unique mechanics and has some of the best loot in the game for when you can fight it, and it's your choice on when to fight it. It has music, great drops, a unique visual, unique mechanics, a great Expert mode change, and generally is the perfect boss.
However, what's the opposite of this? Which boss doesn't have a theme, special visual, unique mechanics, or worthwhile drops that you should chase after?
King Slime. Boring to fight, Expert doesn't change much, loot kinda sucks but Ninja Armour is servicable, no special music, no unique mechanic beyond teleporting to you after a while. And, yes, it is the 'first' boss, but the Dryad doesn't even move in if you defeat it, that's how much of a 'boss' he is. It's also worse because the average new character isn't going to spend their gold and rubies on summons for King Slime when they could fight Eye of Cthulhu or something more rewarding. Slime Rain and random spawns are too unreliable. Slimy Saddle exists too, but I've personally never found it too useful, though that's just subjective.
So overall, King Slime is probably the worst boss, because he's just really boring, not worth fighting, etc. I imagine he'll get his own theme in the next update, and Ninja Armour isn't all bad, Slime Mount is useable, plus his Master Mode pet is cool and can be combined with the Queen Slime one later on. But he really does suck.
If we're talking 'worst boss' just as in we hate fighting it, the worst boss is Deerclops purely because of its Expert mode mechanic interfering with the rest of the entire design of the boss. It's supposed to be kited in a specific way as a homage to the game it came from, Don't Starve, where you fight it in a similar way.
But instead we have random Shadow Hands that can spawn directly on top of the player, forcing them to take damage, get knocked back, and stuck being stunlocked and slowed and possibly frozen. The boss is literally perfect outside of this mechanic.