In Belgium: Tartare or Mayo. Andalouse for the gal, thanks
In the UK: Salt and Vinegar
In both cases: I will have to change seat if you plop yourself in front of me with your poor fries inundated by ketchup. Only the French do that, and it is a Casus Belli in my book.
I haven't seen anyone mention nutritional yeast yet, but that's become my go-to seasoning for almost everything; popcorn, pizza, scrambled eggs, bread, ramen, soft pretzels, and of course on fries. So damn good!
(Yes, I realize the name "nutritional yeast" sounds vaguely unpleasant and unappetizing, but I promise it's incredible if you like savory flavors, and it can also be used as a cheese powder substitute in vegan recipes.)
Ranch dressing, or a mix of half ranch dressing and half ketchup I call "krantchup". Specifically Marie's ranch mixed with Primitive Kitchen ketchup is so good on fries.
I used to work at a sports bar where we made our own burgers and all by hand. Pretty damn good if you ask me.
We made our own Nashville hot sauce that was really fucking good on our fries. I couldn't stop eating them, even if my mouth started burning. Then I started mixing the Nashville with a bit of ketchup...
On a des valeurs en Belgique. Surtout sur l'assaisonnement des frites, la hauteur du col de mousse de la biere, et le caractère optionnel d'un gouvernement de plein exercice.
hong kong mcdonald's is currently serving shaker fries, where you can purchase a packet of seasoning powder along with your fries and shake it all together in a bag.
anyway, my all time favourite (so far) is mcdonald's honey bbq seasoning; fucking delicious. their sakura shrimp seasoning is pretty good too.
i got frozen fries that i heat up in the air fryer occasionally whenever i get a craving; tried some taiwanese popcorn chicken seasoning (i bought a jar of it from a taiwanese noodle restaurant) on it and it's really good.
Rogan josh curry sauce (although most other curry sauces are also good). Chips and curry sauce is a really popular fast food in the UK, but the curry sauce in question is much closer to a brown sauce than it is to anything that's usually associated with the word "curry". So I make my own curry sauce and put it on instead. This does, of course, completely ruin the "fast" part of "fast food", but it is very tasty
On fries I really only do ketchup. I use Sriracha on almost everything else that's primarily potato, but not fries. I used to use A1 steak sauce every so often a while back. I hadn't thought of that in years; might need to go pick up a bottle.