I would love to have a huge blue catfish or something.
Do ropefish count? They get really long but they're not exactly monsters I guess, they don't get very heavy. I love watching mine feed and do their alligator rolls, it's especially interesting when they decide they want the same piece of food. Ropefish are definately one of my favorites.
Hard to decide, there's a lot of monster fish I love, but I dunno if I could pick one as my favorite. I tend to like interactive fish which is why I mentioned ropes, catfish and dats. (Yeah some people don't consider ropes interactive, but a few of mine are with me)