Gardner says the big keeper's progress has been such that he can now be considered "world class" as the custodian closes in on 100 appearances for the club.
"I've been so fortunate throughout my career to play alongside some top keepers," he told safc.com.
I've played with Brad (Freidel) and Joe (Hart) and they're great - Simon is in the same category.
"People say that Joe is the best keeper in the world; I think that Simon's definitely up there with the best and I'm sure other people will agree.
"His saves against Arsenal were immense."