It's easier to start with design considerations and then recommend some brands that meet those constraints.
I'd recommend that you purchase a softbox with a lip so that yout can attach grids and what-not to the front of it. Also, the lip helps contol spill.
I'd recommend a softbox with a removable front face. This allows you the option of lighting with more contrast by removing the front face in the event that you want it. Also, as the fabric ages it discolors. You may never have to worry about it, but it's nice to be able to just replace the front face. A removable internal baffle is also useful for creating even lighting across the surface of the box.
You can get a softbox with a white, silver, or gold interior. Some brands have different colored inserts so you get three in one. White and silver are the most popular and one isn't better than the other, just different. The white ones are less contrasty and the silver ones are more contrasty. Gold is like the silver with the additional property of warming the light coming out of the box.
Size and shape, like interior surface, is a photographer preference. Your needs for shooting motorcycles and automobiles are very different than your needs for shooting jewelry. I like the degree of control I get with large strip boxes when I photograph people, but I also have rectangular boxes in a couple of different sizes for my commercial work. By far, the most popular is a 3'x4' box. It's manageable, big enough for evenly-lit 3/4 length shots, can be used with good effect for full length shots, and is reasonably priced for the size box you get.
General purpose softboxes tend to be square or maintain something very close to a 3/4 ratio of width/length. The other types of softboxes you'll find are octaboxes and strip boxes. Octaboxes are octagonal (duh), whereas strip boxes are much longer than they are wide.
If money isn't your primary concern, Chimera's Super Pro Plus line of boxes is about the best out there. They have a huge range of accessories and modifiers and are very well constructed. They will last you a lifetime through set up and tear down.
Value for the money, Photoflex makes very good equipment. If you're just looking for a softbox with a white interior, the Photoflex boxes are about 1/2 the cost of the Chimeras. For about the same price as the Chimeras you can get a white Photoflex box with silver and gold inserts, giving you three looks in one.
If you're looking for very large softboxes, Elinchrom makes some absolutely wonderful boxes. The quality of light is simply superb. However, some of them can only be used with Elinchrom heads. The Octa light bank is an exception and absolutely kicks a$$.
Any general purpose stand 7' or higher should easily handle a 3'x4' box. You would want something heavier-duty if you have a particularly big box.
Softboxes are a bit of a pain to set up and tear down. Photoflex boxes (there may be others too) have velcro pockets on the front of the boxes that make tear down easier by allowing you to release the rod tension before removing them from the speed ring. Many Elinchrom softboxes collapse like an umbrella, but again you're limited in the non-elinchrom heads you can use.
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