Miners have been put under the microscope by Standard & Poors Equity Research and Citigroup, with positive verdicts on BHP Billiton, Antofagasta and Rio Tinto.In a review of mid-cap miners Citigroup observes that the share price performance of Chilean copper miner Antofagasta has lagged the likes of Kazakhmys over the last six months.Citi has a 'buy' recommendation on the stock, which it identifies as the best investment option among copper miners, thanks to its low-risk production growth profile.For the sector as a whole, Citi says the rate at which metal prices are increasing is slowing down. The US bank still sees the large-caps in the sector as being better placed to thrive in the current economic environment and reiterates that its top pick is Rio Tinto .Meawnhile, Standard & Poors Equity Research has lifted its recommendation on sector leader BHP Billiton from 'hold' to 'buy'.