How good a paper is for use with fountain pens depends. It is usually about how much ink it can take and how it dries. As in will it be obsorbed by the paper or kind of dry on top.
All of this is about aestethics and not always that practical. I usually aren’t into the “best” papers, because they are not practical for me to use because of how long my ink and pen combos of choice takes to dry on it.
If you don’t care too much about how your ink looks on the page. You can just go for really thick paper. It will dry fast and take most ink and nib combos. But purists will hate it for the “feathering”. Feathering is when the ink dries kind of like a feather. It is a sign of the paper soaking up the ink. This used to be my strategy for a long time.
These days I have moved to better papers. But I didn’t really enjoy that experience until my technique made it practical. But the more extreme everything dries on top like Tomoe River is still not something I enjoy.