I would also strongly suggest Onirim for coop in this serie.
the others of the series are not so interesting in coop (or are totally flawed like Castellion coop)
I would agree that Onirim is excellent coop, especially when you throw in a couple of the expansions included with the game.
In my opinion, Sylvion is almost equally as good as a coop as Onirim. The only problem is that Sylvion can be brutally difficult coop. There are certain strategies you can use to win, but it can take awhile to pick them up. Still highly recommended.
Castellion is either broken in coop or I'm playing it wrong and/or missing something obvious because it seems virtually impossible to beat. I've all but given up on Castellion. As a single player game, it seems fine though.