You do realize that AMD bought ATI in 2006 and just kept the ATI name until they ditched it around the time of the HD6000 series, right? So even a HD5970 would be a AMD ATI Radeon, not a ATI Radeon.
If you want to go pure ATI then the last GPU by them before AMD bought them out would be the X1950XTX. You could technically count the HD2900XT but again that was launched after AMD bought them.