Out of the three you mentioned, I think Wendy's burgers are the best. McDonald's burgers, although addictive, aren't really that flavorful, and the one time I had one from BK I thought it was just average.
Now when it comes to fries: I used to love McDonald's fries so much. இ௰இ They were the best; there was nothing like them...until a handful of years ago when they changed their recipe, and now they're just...a sad, bland shell of what they once were. I can't even figure out what they changed; all I know is that something's wrong. :[
Now, when I have cravings for potato, I just go to Wendy's: their fries have improved a lot over the years, and they're very hearty and filling. Although it's not the same as what McD's used to have...