Tough, tough question.
The problem is, even the places with the best fries will vary from batch to batch and day to day. It's all too possible to recommend a place to someone, only to have them get a bad batch and wonder what you're smoking.
That said, I think Steak n Shake's thin fries are great. You have to eat them while they're hot, though. Once they get cool they kind of suck. And I remember a barbecue place in Savannah a long time ago that was fantastic -- cooked the fries in lard for that extra-crispy, artery clogging goodness.
It's easier for me to list the places that are consistently BAD: Checkers, BK,...any place that puts any kind of "coating" on fries.
I like Checkers's fries because the coating and seasoning makes them crunchy and tasty.
My mother's homemade fries taste best to me, but I like KFC's thick potato wedge fries too. The crisp fries the Israelis stuff in their falafel sandwiches are delicious.