I really like late Pre-hardmode (and pre hardmode in general), simply because it allows for the most freedom. You get good movement, and you can do anything in any order at that stage of them game and still have it remain viable. Endgame progression kind of sucks, but Duke Fishron, Empress of Light, and Moonlord are some of the best bosses in the same, so that balances it out. Early hard mode is pretty fun - testing out new weapons and such, and fighting the mechs and Queen Slime with said weapons is also a treat.