Fortune Cup + Dark Signers rivals anything in the original series, even though I sort of divide 5Ds into those 2 arcs and everything after.
GX had the theme deck frenzy, which in turn lead to some great duels, but that was it. To beat my own personal dead horse; identity crisis plot with a singly sympathetic character.
The original series had its own demons, namely struggling with keeping Honda/Tristan and Anzu/Tea relevant and Atem's sudden mellowing, but that the series survived the shift in tone, even if some people have a "Dragon Ball vs Z" sort of relationship with it (I have a friend who loves pre-duel monsters, eh on duel monsters, for example), is part of what made it fun.