Velcro is hook and loop fasteners
There is a whole list of these on Wikipedia https://en.wikipedia.org/wiki/List_of_generic_and_genericized_trademarks
If you're in the south, Coke. Which is stupid, because it takes an extra few sentences of interaction to convey what you want.
"I'll take a coke."
In any other place, when you say coke, you mean coke.
Surprised noone said photoshop yet. I mean we literally have /r/photoshopbattles.
Wite-out. It occurs to me that not only do I never use the generic term, I actually have no idea what it even is.
Here's a little-known one, unless you work the in warehousing/transportation/distribution sector:
It's a term used for basically a big ass cardboard box for shipping bulk items. They were originally produced by the Gaylord Container Company, which doesn't exist anymore, but the name has since become the generic term for those big pallet-sized boxes.