Billiat, who relinquished the PSL Player of the Season award to Lebogang Manyama last term, faces an uncertain future with his contract at Sundowns set to expire in June.
The Zimbabwean, however, has options, as Orlando Pirates and Azerbaijan champions FK Qarabag are among the teams reportedly coveting his services.
Castro spent two-and-a-half seasons with Billiat at Masandawana before moving to title rivals Kaizer Chiefs in the January transfer window.
And the Colombian says he saw first hand just how good the playmaker is.
"For me the best player in South Africa at the moment is Khama Billiat," Castro said in a video shared on Twitter by Chiefs.
"He's an exceptional player. I was training with him every day. For me this guy is the best player in South Africa."
Meanwhile, the 28-year-old says he is settling in at Naturena as he awaits his debut under Amakhosi coach Steve Komphela.
"I feel happy now that I finally got an opportunity to be here. I need to work hard to help the team. So everybody, they give us new players a great welcome, they look like family here."
The Glamour boys trail log leaders Downs by five points after 17 games, and next visit Baroka on Sunday afternoon.