Yeah it's Schumacher without a doubt, he really led then to glory and without him I don't think they would have recovered from their drought and possibly not even be the most successful team in the sport. Apart from him, Ascari was a brilliant driver, he dominated 52 and 53, again I think without him Ferrari wouldn't have found their feet so early in the championship. Those two I think are the best, after that, Lauda and Villnerve were both excellent driver and recently Alonso has joined that list.
I can see the argument for Schumacher he definitely piled on the numbers as far as GP wins and WCCs/WDCs go and was certainly a big part of the Ferrari revival. But I must add that in my eyes some of his victorys will be tainted by his tactics.