Any tomato variety will work as long as you pick them while they are unripe. The trick is to wait for them to grow to their full size but pluck them before they fully mature and start turning from green to yellow or red. Just don't confuse heirloom varieties that are actually supposed to be green in the first place. Those will still work for fried green tomatoes, but you need to squeeze them to make sure they are still firm and unripe so they don't fall apart while breading and frying them.