While he's a great player, there's a few things stopping him being one of the greatest of all time. Because there's so many great players that are classed as "all time greats", you need to be someone really special to stand out even from those. When you think of great French footballers, at least for me, Cantona is a name that I'd come up with but not before Zidane or Platini. In terms of great strikers of all time, Cantona would be before the likes of Van Basten, Ronaldo, Baggio. And of course, the average football fan would probably tell you that Zidane, Pele, Messi and Maradona stand out as the best players ever. So in that regard, Cantona will fall short of being considered in the very top tier.
Having said that, anyone that watched Cantona in his prime knew how amazing he was. I'd consider him to be comparable to Christian Vieri - a player considered among the best in their position when they were at their best and people will acknowledge them as great players. But for one reason or another they won't make the top of many people's list - usually the fact that they're overshadowed by another.