I don't like Ladislaw much - he's nice enough, but very shallow, and I could easily see him turning out to be a Nice Guy (TM), but he makes Dorothea happy and she deserves that. I think the "perfect" marriage would have been Dorothea and Lydgate, which is never floated as more than the faintest of could-have-beens (by the time they meet Dorothea is already engaged to Casaubon, and Lydgate is too concerned with appearances to realise it anyway), and then Eliot spends the rest of the novel showing us how things turn out more or less OK for both of them despite this (and Dorothea is still the person who turns Lydgate's life round for him).