McDonalds...free large fry with any purchase. I'll either get a drink or mcchicken and spend less than $1.50 for a whole meal.
Taco bell sometimes has deals but the big thing now is their reward system is really good. All you need to do is spend like $16 worth of points to get a free nachos grande or chalupa reward.
Wendys is also good too. Haven't really used Burger King's