The biggest marlin ever caught record is slightly sketchy and there are certainly many reports of fish that would easily beat the record by up to a whopping 1000 pounds that were lost next to the boat, never recorded or that swam off with the camera etc.
Largest marlin caught on rod and reel
The biggest marlin caught on rod and reel that I can actually validate with some form of evidence was caught in 1970 in Honolulu and weighed in at a whopping 1805 lbs (or 820kgs for people like me)! It is pictured below and it is certainly a monster (click to enlarge).
Largest commercially caught marlin
The largest commercially caught marlin bottomed out a 1000kg scale meaning it weighed in excess of 2200 lbs! It was apparently caught on a handline which, if true, is one amazing feat - it is pictured below (click to enlarge).
Big Marlin Video
Below is a video of a 1026 lbs marlin and although it isn't a record it is one impressive fish!