Uh, okay. Taking the Rise of the Empire era as defined by Wookiepedia, starting about 1000 BBY, I'm going to shoot in the dark here and say that it's a trick question. In 26 ABY, one of the oldest worldships was less than a thousand years old. (Thanks, Wookiepedia!) So I'm going to hazard an answer of...
The Yuuzhan Vong.
*Winces in anticipation of being hilariously wrong*