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Leopard.

Seriously, that question can't really be answered because we don't know what kind of machine you have. With a new Mac, it'd be hard to recommend Tiger...Most people today have no problem at all with Leopard.
 
All else being equal, Leopard for sure. Any of the 'early-adoption' bugs with Leopard have mostly been worked through, and its got tons of features that set it apart from Tiger.

Do you have any specific reason that you'd want to run Tiger?
 
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