Back in the early 1990's a well-known magazine listed their top 10 toughest freshwater fish based on a variety of factors such as toughness, accessibility, size, etc..
The results were as follows: 1 (tie)-Amazon Catfish and sturgeon, 3- The Masheer from India, 4- Nile Perch, 5 New Guniea Bass, 6-Taimen and Huchen, 7-Wels , 8-Pirarucu, 9- King Salmon, and 10- Alligator Gar.
I have battled a few of these monsters, but not all. So far here is my top 10 list
1- Rare Jullien's Prized Golden Carp, 2- Golden Dorado, 3- Siamese Carp 4- Amazon Catfish, 5- Tigerfish, 6- Mekong Catfish, 7- Chum Salmon, 8- Peacock Bass, 9- Giant Snakehead, and 10-Pacu
A few on the list I hope to do battle with someday soon are the New Guinea Bass, the Wolffish, and the Masheer.
What are your top 10?