Answer:
On balance, near the equator, but I would say it really depends on the environment you were looking in. On land, relatively little lives near the south pole as it is an ice sheet, and there is no land at the north pole. Marine environments are less clear cut. Tropical reefs have high biodiversity (probably more so than any other marine environment), but generally everywhere life is sparser in the deep ocean. Some polar marine environments also contain abundant life though as the cold increases oxygen solubility in the water and also means most organisms have a slow metabolism. Glacial activity grinding up rocks also means polar water can have more available nutrients.