Quite simply put, Arches Watercolor blocks are the best watercolor paper available anywhere.  I use all the different varieties available - the cold press versions, particularly the rough surface, are a fantastic surface for any kind of drybrush techniques - the rough texture adds all sorts of possibilities for the application of paint.  If you are doing more washwork, you may want to consider the hot press variety.  It has a very smooth, untextured surface, is less absorbent than the cold press and will take marvellously detailed images. Because its less porous, it also extend drying time, so you can do more color manipulation after application.  Buying your paper in blocks is a good way to improve your work flow.  No stretching or taping paper to a board - its all ready to go in the block form - just separate the cover sheet from the rest with a utility knife, and you are free to paint!  When your work dries, any warps or buckling will work itself out as the block stretches the paper back to flat!  When you have finished a painting, simply cut the top sheet away from the block and you have a pristine, fresh sheet of pre-stretched paper.