I like Skeletron, in expert and master. Very few of the prehardmode bosses present an actual challenge, while Skeletron forces you to actually prepare a solid arena because the boss can cover a lot of ground quickly and create a lot of projectiles, and you have to actually plan what you will do and create a strategy, which doesn’t really apply to other prehardmode bosses as much. In Normal Mode however he’s a complete joke and a pushover.
I also actually like the Lunatic Cultist, despite how easy he is. The attacks are reasonably predictable and reactable, and the way the boss plays is different from most other bosses. Give him a reasonable buff and he’d be my favorite.
Duke and Empress are almost everyone’s favorites. I personally like Empress more because the battle feels intense and chaotic while still being fair and learnable, but Duke Fishron is a great battle too because it has a unique emphasis on mobility and forces you to pay attention both to the boss’s and the sharknado’s location.
Although I don’t like him as much as the other three I listed here I’ve appreciated Moon Lord more and more as I’ve played more. First couple times he just seems like an impossible spammy hellhole, however as you learn the boss you learn generally when he uses each attack and how to respond to them (phantasmal eyes - you generally want to watch the side of the screen and weave outward and upward if you see a giant wave of them, for example), and the boss is actually a pretty fair fight now that I have experience. Perhaps he should be toned down a bit for the sake of newer players, but then again he *is* the final boss.