I would personally say Cyrus, though N and Ghetsis are very close seconds. All three are both done very well from a story perspective, are competent enough to nearly achieve their goals, and are on the harder side to defeat in battle. N especially, since he has a legendary on his team.
I like Archie, Maxie, and Lysander from a story perspective, but their Pokemon teams are quite weak, so actually fighting them feels like a letdown. I haven't fought any of the team leaders in Alola/Galar/Paldea yet so no comment there.
So I guess the worst would be Giovanni. He doesn't have much of an actual evil plan in the games beyond "be a crime boss", and most of his reputation is from stuff he did in the anime or manga. His team of Pokemon is among the easiest to beat, almost on par with Gen 3 Archie and Maxie. I have not played through the Rainbow Rocket stuff in USUM, so take my opinion with a grain of salt, though. It's possible I might change my mind after playing that.
That being said, if you count Archer as a team leader, which I don't, he's at the bottom. No contest. His incarnation of Team Rocket doesn't do anything besides cut off Slowpoke Tails, get their HQ raided by the player and Champion, and their grand plan is to call Giovanni for help, but he never bothers to show up even though the radio broadcast does go off successfully. To top it all off, Archer's Pokemon team is a pushover with only three Pokemon, two of which aren't even evolved.