We can argue all day about what constitutes true love, but all we can go on is what the writers have given us, and so far, Bell is Rumpel’s true love. He even admitted that she makes him want to become a better person. Cora will never be able to inspire that in him becaue she’s just like him. They’d end up destroying each other (which, oddly enough, is exactly what’s happening).
But it won’t matter in the end anyway because she’ll probably be killed off in this Sunday’s episode.