Strathmore 400 series Mixed Media paper. It comes in precut pads and doesn't have to be pretreated (no acid bath or size required) to give good results. It's also easy to find and pretty cheap. It can be subjected to multiple washes and won't fall apart (so toning is easy). I've used it for 4 color gum as well (needs to be sized for that). It's a wood fiber paper, so it won't be as archival as a cotton fiber based paper, but it's good for learning on and will probably last a few decades without deteriorating noticeably. It also has an extremely smooth texture, if you're into that kind of thing. It's not the best paper I've ever used, but it gives a lot better results than a lot of the more expensive water color papers I've tried, and with far less effort. It's what I learned on, and if I were teaching someone else alternative process photography, it's what I'd start them out on.