Shadowverse

Also, since we're talking about the best cards: don't you think that the best legendary card from the expansion is PDK? It created a whole new archetype, which I personally encounter much more often than the usual ramp, and it also made some old cards playable again (Barmaglot, fire lizard). On the other hand, the strength of Sylvan lies not so much in Cassi, but in the new spells and amulets (although I haven't encountered Sylvan often enough to judge adequately).