In the most literal sense? Hmm, I'd say one that can keep a Powder Blue Tang, or something of equivalent difficulty (Regal Angel, Anthias, SPS, Clams, that kinda stuff) alive without a huge amount of user intervention, all matters taken into account (size of tank, style of tank, etc).
Also, a tank that can hold its own against problem algaes gets a vote in my book.
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This and that.
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