Updated: Sep 13, 2021
I'll just put my personal opinion out there. There are so many brands to choose from and the quality and texture vary. I believe it is a very personal choice, I love trying new things and the paper I use depends on what I am planning to paint. I have a whole blog post dedicated to texture right here.
If you are buying for the 1st time and have literally no clue and are a bit overwhelmed by the choices like I was: I suggest going to a store and seeing if they have samples you can feel. It will help you decided what you want and don't be afraid to choose a few kinds to experiment with. The biggest thing to look for is to make sure it is acid-free so your paintings last a long time without fading or breaking down. There is a nice long article from Strathmore here about the difference between acid-free and archival paper here.
Big brands like Winsor and Newton, Strathmore, Arches, Fluid 100, etc... are good well known reliable brands to try out in my opinion. Another one I discovered, later on, was Bee Paper which I like a lot.
Watercolor is made of 100% cotton or wood-pulp. 100% cotton is stronger and can hold large amounts of water from my experience. Wood-pulp is a good inexpensive alternative to 100% cotton. However, it tends to warp a bit more easily and not absorb water as well.
Usually measured in pounds. The higher the weight the more water and paint it can handle before showing signs of stress -- aka warping/buckling. The most common weight used is 140lbs. It can handle a fair amount of scrubbing or abrasion from a brush or eraser if you sketch on it. Anything below 90lbs will have a hard time handling the water and not withstand much abrasion, which can change the texture of the paper. 300lb is super thick and will be more than enough to handle large amounts of water.
They are loose and separate from one another. They're usually cheaper, but that doesn't mean they aren't good quality. I like using them for individual pieces of art. I don't have to worry about cutting it, tearing it on accident, or removing gum or whatever was holding it together.
One big roll of watercolor. OK, maybe not as big as the picture shows... However, if you want to do huge pieces or not be restricted to any size this is a good option. You will have to cut, flatten, and stretch it out yourself.
Pads or Notebooks
Watercolor pads usually have one side bonded/bound. They can be bound several different ways; It could be gummed, glued, ring-bound, etc... This is great for traveling. And if you like organization they look pretty neat.
Use blocks if you are super brave. It's a love-hate relationship at this point. I love blocks because two or all sides are gummed or glued. This keeps the paper from warping/buckling which is amazing. However, it strikes fear into my heart.
Removing the page from them can be a challenge and not a fun one. Some require a knife in their instructions for removal. *eyes widen*.
There's a high risk for tears in my experience, but note that some brands are easier than others. *whimpers "what doesn't kill you will make you stronger"*
When you do get the page off successfully sometimes glue or whatever was used sticks to the edges of the paper.
Get to it!