The right-back faced Costa in Arsenal's 2-0 victory over Bayern in the Champions League earlier this season and identified the Brazilian as a particularly tricky customer despite the result and clean sheet.
Costa went on to torment Mathieu Debuchy in the reverse fixture at the Allianz Arena in a 5-1 victory for the German champions.
"Probably Douglas Costa," Bellerin told Amuse when asked about his most difficult opponent in senior football.
"You make mistakes in a game, but if you come away with a win, that's all gone. The result is all that matters, and we won that game."
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Bellerin believes that the win over Bayern in the group stage proves that Arsenal have the ability to knock out holders Barcelona in the last 16.
"I played for Barcelona for eight years, so it's a massive thing for me and my family, who have been watching me since I was young," he added.
"The day the draw was announced I started getting texts and calls from everyone. It's crazy how a game moves so many people.
"We're very excited – it's going to be a hard game, but if we did it against Bayern, then we can do it against Barcelona. We have a great team mentality and our confidence is very high, so we fancy anyone at the moment."