I disagree...medium heavy at a minimum....a 20 pounder will own your a$$ with a medium and it'll be just that much longer to play the fish out while a rock dog decides it'll move in on it to slurp it down like a sardine...or Harvey the Happy Harbour Seal decides he'll take it off your line. Mediums do not have enough backbone to take it off the release clip when it is buried in the clip...and reeling in just a flasher is even painfully slow. In the Shimano line Techniums or Convergences.., the difference between the medium heavy and heavy is rather subtle thing....especially when paired with reels that have a large cork drag system like an Islander, Trophy, Etc...
A medium heavy is the most popular...and the heavy adds extra power especially when you're pulling in the larges....