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I like Duke Fishron. I remember fighting him on mobile at least 50 times because I wanted the Fishron Wings for me and my sister. I recall setting up a long asphalt track, and I just grinded him over and over. His drops were the best for mobile too. The bubble gun was overpowered, and the Razorblade Typhoon was awesome.
It's a hard choice... I haven't really defeated all of them yet so i can't really form opinions on all of them.
Wall of flesh was definitely the most interesting one so far for me... i just don't like the requirements to summon it. (i get very emotionally attatched to the npcs, especially the guide, i mean, the guy's with you since the very beginning then you just straight up sacrifice him.)
I do enjoy king slime though, i love messing with the slime hook. Also slime rain can be a fun event when you're up for it.
I really like the Wall of Flesh. It's annoying to prep for, but it's such a unique experience fighting it compared to all of the others. It dictates the terms of the fight, forcing the player to constantly retreat while avoiding its attacks and fighting back.
I enjoy bullying the Destroyer. Shredding him as he uncoils is one of the most satisfying things in this game, and he's my favorite to grind because he's the easiest boss that can drop dev set pieces and the crafting materials for his summon are super easy to come by.