100% cotton laid paper, preferably with a full deckle edge. The “laid” part is important, the wire mesh from the mould leaves a texture (like a watermark) you don’t get with modern “wove” paper.

There’s also quite a lot of handmade rag paper out there, most of which is nice. Just be careful of the ones with embedded flowers and stuff if you’re using a laser printer, they can come out and stick to the rollers. (Also much of this is terrible to write on with a fountain pen, as it’s not sized to stop the ink bleeding.)