Mx. Redzly
Spazmatism
I made a poll about this many years ago, but I think its a good time to make a new one as Terraria has been updated a lot ever since. Remember to consider difficulty levels (normal, expert, master) when choosing your boss! Event bosses and Mechdusa arent here sadly.
For me, Skeletron is actually the most frustrating boss in expert/master mode. Im never able to dodge either his homing skulls or hands and I need to fight him a lot of times until I beat him, plus the fact that you need to wait a day or two to refight him if you are not on Journey Mode. And so I could also think of Skeletron Prime, but ironically he isnt as frustrating to me during the time I fight him on hard mode as normal Skeletron.
For me, Skeletron is actually the most frustrating boss in expert/master mode. Im never able to dodge either his homing skulls or hands and I need to fight him a lot of times until I beat him, plus the fact that you need to wait a day or two to refight him if you are not on Journey Mode. And so I could also think of Skeletron Prime, but ironically he isnt as frustrating to me during the time I fight him on hard mode as normal Skeletron.