Langerak spent five years at Dortmund between 2010 and 2015, before eventually signing with his current side Nagoya Grampus in Japan.
"Ter Stegen is crazy the way he can make some saves," Langerak told Goal . "He plays fantastic with his feet, and when he plays at Barcelona this is how they need to play.
"Alisson also, at Liverpool, is somebody who makes unbelievable saves and is very strong for the team.
"I think so far in the Premier League they have 19 or 20 clean sheets, so he’s been unbelievable."
Langerak has also reserved praise for the shot-stopping pair in the last-four tie between the Dutch and English sides.
"For me I also like Hugo Lloris, he is a fantastic goalkeeper who makes amazing saves and he is somebody who I like to watch a lot," Langerak said. "And he has had a fantastic career, he is an amazing goalkeeper.
"[Onana] also fantastic, he fits in very well into the way [Ajax] play.
"So four very good goalkeepers, and it’ll be very exciting to see who comes out at the top at the end."
Pressed on a prediction for who will advance, Langerak has tipped the competition's two remaining English sides to meet in an all-Premier League final.
"It’s difficult, I think my guess is like anybody else’s guess, but I think Liverpool has a very good chance, as they are in good form at the moment, but they need to catch Barcelona on their bad day," Langerak said.
"For anybody to beat Barcelona you need to catch them on their bad day.
"And Tottenham–Ajax, it’s also very difficult. Ajax is a fantastic young team, they are doing very well, scored most goals in Europe so far this season.
"So I think it’s difficult, but Tottenham also have good quality players. So, I’m guessing, maybe Tottenham and Liverpool."