Perhaps the most consistent brand on the market are the ones made by Derek Hersey & company: Unison. Hand-made in the Northumberland area of England, these well formed pastels offer a full range of colors in two sizes, regular and large. They measure 2 1/8ths" long by 5/8ths" diameter (54 mm × 16 mm), or 2 3/4" x 5/8ths", respectively. My research has a "full set" being 390 sticks.
Our friends at Dakota Pastels rate Unisons as the fourth softest brand, under Schmincke, Great American, and Sennelier.
I definitely include Unisons in my own list of very soft pastels, but one always knows what one will get for handling: perfection. The flip side is the lack of very intense colors. I am more often surprised when a hue is intense. Some of the greens are, and the middle value red is fine. Lighter ultramarines are clear, but mostly they are valued for their grayness, in my opinion. More intense colors, for my part, are found with Diane Townsends or Senneliers. The review at Wet Canvas differs.
If you value the full range of expression with colors, from grayed-down to full chroma, you have a great start with Unisons. The brand has done a fine job of asking it's hues to remain true to base pigments, with little reliance on white. Hersey prefers blended hues, rather than the tint and tone route that creates thoroughness, but lacks a natural basis. I would say an artist who must refer to nature has a great ally in the Unison line.
As with most brands, they are boxed attractively in many types of sets, but I would beware of being too enamored of some sets if you are going to be a serious pastel artist. What I mean is, the landscape or portrait selections will be a selection made by someone whose choices may baffle you, and you'll buy a few of these (expensive) sets and be more or less stuck with colors you don't want, or repeats of many colors.
My advice is to get color sets, because you will enjoy the necessary ranges of values in a given hue. An economical way to get complete sets of colors may be to buy sets of half sticks (other brands, here), or try Terry Ludwig's Maggie Price Set which is a complete (more or less) set of hues, each hue having six values. Then, fill in with open stock.
The Red Corner work below relied heavily upon Unison greens for both the subtle greens in the foreground that counter-pose the intense reds, and also for the green highlights there. The number 2 Red Unison probably played a big part in this work, as well.
See their well presented color charts online, beginning here.
The best blogger review of Unis is Katherine Tyrrell, and access to these reviews is best made by going to her Squidoo Lens, Pastels, and page down to reviews. She offers six reviews of her home brand at first, but page down and catch two more reviews of dark and light sets of Unisons.
Alistair Boddy-Evans, at About.com, also has much love for the Unis. See his interesting review of eight favorite brands here.
Hersey tells his story here.
I would like to highlight the delightful work of new blogger Carolyn Bannister, who posts some still life work done with Unisons here and here.