To find out which are the largest carnivores of all time, you should look to the sea. The largest carnivores of all time are in the sea.
The largest carnivorous animal would be the blue whale, most likely. It is not only the largest carnivore of all time, but also the largest animal of all time at 100–120 feet long and >50 tons. They eat several tons of food per day, including crustaceans like krill.
The largest shark of all time would be the Carcharodon megalodon. He preyed on dolphins and whales. Unfortunately, the exact size is not known because all that remains of Megalodon sharks are teeth. Scientists have not discovered complete skeletons and possibly never will.
This shark had extremely large teeth that were almost 8 inches long. The largest teeth in the largest great white sharks are <3 inches long, while tiger shark teeth are about 1 to 2.5 inches long. Here is a side by side comparison.\
Based on these tooth deposits, some estimates put it around 33 feet, while others put it around 60 to 70 feet long. What can be said for certain is that Megalodon was significantly larger than any other species of shark and any land carnivore that ever lived.
At an estimated 40,131 lbs-sq in, Megalodon also had the strongest bite of any predator that ever lived. (Just for reference, this shark had a bite that was over 5 times that of Tyrannosaurus rex.)
Finally, the largest sperm whale species of all time is Livyatan melvillei. Livyatan was 39–54 feet long. This probably preyed on other large whales like baleen whales.
This whale may have been a predator that competed against Megalodon for prey and territory.