I'd say Pan for sure. He was the most powerful and evil villain we've seen so far, I'm not sure any upcoming villain will be as powerful or evil as him. They'd definitely have to bring their A game!

Most definitely

I'd pick Cora to be second, then Zelena. It was tough for me between Cora and Pan, they're both so powerful. I tried to edit this into my comment above but it wouldn't let me...not sure why :/

Cora or Pan.

Zelena for sure, it's almost confirmed by the show when Rumple himself said she's the most powerful witch he'd ever met even more powerful than Cora. And also it was quite impressive (and weird I dunno) how she was powerful enough to actually poof Glinda to another realm ! (isn't that impossible ?)
In my opinion Zelena had so much more power than what we've seen

Honestly, I think Rumple was just saying that to make Cora mad. She "broke" his heart, he practically hated her. Think about it--she'd hate it if HE of all people, said there was someone more powerful than her. And besides, Rumple isn't the most powerful. I'd say Pan, Cora, Zelena THEN Rumple.