De Gea has enjoyed a superb start to the season, having kept seven clean sheets in nine appearances in the Premier League and Champions League, while Buffon continues to defy his years at Juventus.
Darmian says that 39-year-old Buffon is the greatest goalkeeper of all time, but the versatile full-back feels that De Gea is currently in the better form.
The 27-year-old told the club's official website: "I know David very well and I think, at this moment, he is the best goalkeeper in the world. He is very important for us and I think that save at Stoke [from Jese Rodriguez] was incredible.
"I think Gianluigi is the best goalkeeper of all time. He's also a good guy. So I am lucky to play with two goalkeepers like this.
"Of course, it's close [between the two]. I don't know. I think David is the best goalkeeper in the world and Gianluigi is the best goalkeeper of all time. I think they are similar because they are very strong and they make very good saves. They also have good personalities."
De Gea, who has been with United since 2011, has frequently been linked with a transfer to Real Madrid.