Cox restricted Coleman Medal leader Ben King to just 1.2 from only seven possessions in Fremantle's 51-point thrashing of Gold Coast in Perth on Sunday.
It continued a strong run of form from Cox, who often flies under the radar given the hot form of skipper and All-Australian contender Alex Pearce.
Longmuir wanted to highlight Cox's worth after the 11.14 (80) to 3.11 (29) victory, which extended Fremantle's winning streak to 14 matches and put them two wins clear on top of the ladder.
"Brennan Cox is probably the most underrated key back in the comp, and probably has been for a while," Longmuir said.
"His job, not only was he able to negate King, but I thought he was able to win the ball back a lot, and especially in that last quarter,
"Our backs are working really well together, but they're getting good support from up the ground as well."
Murphy Reid (28 disposals), Andrew Brayshaw (29) and Shai Bolton (26) were all prolific ball winners against the Suns, while Michael Frederick kicked an equal career-high four goals.
And in another huge boost for the high-flying Dockers, ruckman Sean Darcy made a successful return to the senior side in his first AFL appearance since round five.
Darcy's stats were by no means extraordinary - 20 hit-outs, six disposals, two clearances and one goal.
But the fact he made it through the match unscathed was noteworthy in itself, with Fremantle hopeful Darcy's fitness and form will build further over the coming weeks.
"I thought when he was able to get his feet set around his ruck contests, I thought his strength was good," Longmuir said of Darcy, who has battled concussion and calf issues this year.
"I thought he moved around the ground well. It was nice to see him finish off his work in front of goals.
"It shows the real thrill that the team had having him back in the team, the way they celebrated that (goal). He's only got to build on what he did today."
Darcy's presence, plus the easy nature of the win, allowed Fremantle to rest No.1 ruckman Luke Jackson for periods of the final quarter.
Fremantle will be aiming to make it 15 consecutive wins when they take on 14th-placed GWS (6-9) in Canberra on Saturday.
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