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You asked about "the best"... Without question.. Most bang for the buck, so freekin flexible in it's way you can use it.
The LastoLite HiLite. Available in 2 sizes.. I recommend the larger.
I bought one of these 2 years ago and although a bit fidgety for setup and put away, this thing rocks hard all day long. Combine this with a white train.. (I used a roll up solid window shade I glued velcro to) and it's a total high key portable/in-studio solution.
I have even placed garments on it and stood over top of it to shoot products. I've gelled it to alter the background color, I've used the reverse side to get a black/grey, I've placed it sideways to get lay down shots, I've used it as a HUGE softbox natural light window replacement, I've hung it over motorcycles to light them..... It's like a pair of light 'vise-grips'
Run.. don't walk to get this in your kit. A bit pricey, but nothing compares to this.