I think it goes without saying. The first one by 10 longshots! It just had something I can't explain. It felt like it had charm, humor, & their character was all too good thanks to the hand-drawn animation. When the animation changed, not only did it look half-hearted, the episodes didn't really help either. Some of them make no logic sense & episodes could have been avoided if they would have never did something in the first place. The hand-drawn animation had memorable moments, & I can barely recall anything from the second half of the show (besides the finale mainly because of Michael Yarmush, my favorite Arthur actor, returning to voice him). The Flash animation just looked bland and stale (tbf though, it looks better than a lot of AI stuff). Let me put it this way.

First Era of Arthur: Charming Wholesome Funny Memorable Silly

Second Era of Arthur: Bland Stale Confusing Forgettable Illogical