The former Tottenham Hotspur midfielder has thrived since re-joining Swansea this summer.
The Iceland midfielder has shone since rejoining Swansea this summer, after spending two seasons at Spurs.
And Monk feels Sigurdsson is good enough to play for any of the 'top sides,' suggesting that his best quality is the amount of running he does off the ball.
"Gylfi has been under-rated and could play for any of the top sides. But he feels comfortable here, likes the way we play and we allow him to express himself," Monk is quoted as saying by the Daily Mirror.
"It's about signing good players to fit with our style and he matched that. That's why we pursued Gylfi throughout the summer and he hasn't let us down.
"What many don't see is how hard he works as well. He doesn't stop running and doesn't get enough credit for that.
"His work-rate is infectious. When the other guys see someone working extremely hard they know they have to put in a shift."
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As Opta stats show, the 25-year-old has shone so far this Premier League season, registering more assists than anyone except Chelsea midfielder Cesc Fabregas, while also creating 18 chances for the Swans this term.
During his time at Tottenham, however, Sigurdsson made just four assists, going a whole campaign without registering one in his second season at White Hart Lane.