What is the most important item in watercolours? Is it the paints, paper or brushes? Personally I agree with the artist and teacher Charles Webster Hawthorne in his small book 'Hawthorn on Painting" ` in fact a collected edition of his students notes, published after his death in 1930. He said "buy good paper- fifty percent of and more of your watercolors depends on the paper you use". Actually it's the hand that guides the brush but good paper makes it easier.
Most starting off in watercolour use a cheaper paper with the British Bockingford being the most popular, at least in the UK. You also have a number of cheaper papers in the German Hannemuhle range, while in the USA Strathmore have cheaper papers. And there are others. The thing about these budget papers is that they are not made of cotton, but high quality wood pulp in the case of Bockingford and I think cellulose or alternatives are used in others. I have tried some that Great Art sell but wasn't impressed although the 100% cotton Centenaire is a good paper.
I stress the following is just my opinion and others may well differ. Also I haven't tried every available paper although I have sampled a good number.
This is my current favourite paper in the 'High White" version, also in a block although blocks are more expensive than sheets.

Fabriano Artistico "Extra White' was one of Charles Reids favourites and also mine. He said it suited his style of painting, and didn't like Arches as he said it was too 'hard -sized"
I've gone off it for three reasons. Firstly the price has escalated, something that has affected all papers in line with the increasing overall costs for watercolour artists. The others are the block size "18 x 12", my preference being "16" x 12", and the way the blocks fall apart when you get down towards the last few sheets. Waterford hold together very well.

This paper was introduced to me by Judi Whitton and Charles, who knew Judi who attended at least two of his workshops, one of which I was on, probably heard of it from her. He considered it equal to Fabriano but particularly liked the block size - 40 x 50cm. Unfortunately getting hold of Schut papers, including the favoured Noblesse, was difficult. I first read about it in a book by the late Zoltan Szabo. I actually obtained some from an art shop in Stow, then from a small wholesaler, actually a private house with a small warehouse, in Banbury. This then dried up.
I recently decided to see, with rising prices in papers, if Schut was still available and at what sort of price. Enquiries discovered the Schut paper mill had changed hands and was now part of the Fontaine group. Further enquiries finally brought the information that ALL 100% cotton papers under the Fontaine label were actually the paper previously marketed as Schut Noblesse. There are four surfaces and a good range of blocks and sheets. A new surface called 'Cloud Effect' is also available.
Both Great Art and the SAA are selling them, not yet Jacksons though. There is some slight confusion in that there is conflicting information regarding the number of sheets in a block, some saying 15 and others 25. This has an effect on the price per sheet, which seems to be approximately the same as Waterford or slightly higher. I haven't bought any as I have a large supply of paper, possibly enough to last except if I reach 100. Actually I'm exaggerating here slightly. I'm currently painting on the reverse side of failed paintings. Ron Ransom told me he did this and it is perfectly possible on decent papers like Fabriano and Waterford. I have many!
What other papers have I tried.? Quite a few actually. I have a small stock of Moldau, the Czech hand-made paper, obtained with great difficulty, the saga of which is related somewhere on this blog. A lovely paper but getting hold of it oh dear! I also found, quite by chance a small shop selling quite small sheets of loose watercolour paper in Amalfi on the Sorrento coast of Italy. The largest was 11" x 15", quite light at about 90lbs. A lovely paper I have to say but I I've not been back since that second visit. The lady owner, when asked where she got it, waved her arm saying 'at my factory over there'. I doubt I'll go to Sorrento again but would certainly buy more if she was still there.
Another paper creating waves, especially in the USA from where it emanates. is Stonehenge Aqua. Rave reviews have appeared from American artists. Jacksons sell it and I've purchased and painted on a block of the 16" x 12" size with a not surface and rate it equal to the Waterford. Certainly not superior (in my opinion). The American Strathmore group have a good name but I have no experience with any of their papers.
Other than that what can I say? There are lots of other papers including some very expensive hand-made ones. The ones I've listed are mainly 100% cotton, which to me is a benchmark. Cotton and linen is mentioned in some of the high end papers.
If price is a factor, and when isn't it these days, for us struggling amateurs, Great Art do an own brand called Centenaire which is 100% cotton and cheaper than the above. The Hannemuhle group have several cheaper papers and there are a few others. Frankly the long established Bockingford, made from high grade wood pulp (they say) is as good as anything if you want an inexpensive, decent paper. Some will say cotton papers are better, and they probably are, but do many amateurs need to spend big bucks? Based on my experience with several local art groups I suggest no is the answer.
Added 10.15pm. I omitted to mention Khadi, a Nepalese hand-made paper, available in a wide range of sizes and weights. Some of my fellow artists have tried it with mixed responses but it is 100% cotton and very reasonably priced. It isn't a favourite but I do have some and occasionally use it. The surface is on the rough side whereas my choice of surface is not.