Wii U vs Vita vs Xbox

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Sometimes, for various reasons, a system simply fails to attract a large audience. Nevertheless, among the few who do buy them, such systems are sometimes quite beloved, as seen with the Dreamcast for instance.

Of the lowest selling major systems from each of the big three, Wii U for Nintendo, Vita for Sony, and the original Xbox for Microsoft, which do you consider to be the best as a system, and what about it makes it worthy in your opinion?