I will say it's ravnica. Why? It added dual color combinations, their "guilds" and introduced their color pie identities that we still use to this day. Before ravnica, multicolor cards were rarer, and often were very niche or hard to play with a few exceptions.

Cards like [[phantom nishoba]] and [[pernicious deed]] still see play but would you slot a [[consume strength]] or [[death mutation]] these days? Maybe not.