It depends on how you want to measure it. You would need to add more qualifiers to get the list you seek. Are we talking production? Volume of Exports? Value of Exports? Number of acres available for agriculture? Do you consider Russia as Europe? The Ukraine? Are we talking the largest organic farming nations or traditional farming nations?
If we are generally talking about exports, then it is the Big Four - France, Germany, Italy and Spain. If we start talking dollar value, then the Netherlands and Poland start coming into the top four instead because of the more expensive nature of their exports (meat, flower bulbs, etc.).
Other answers and variations of the list depend on which qualifiers you choose.