My Favorite would have to be the 41 Magnum. I am not sure the reason but I really like shooting it and reloading for it. Its such a great overall round I am not sure what it never took off. I guess its because of the factory ammo listings for it. I don' think I have shot a factory round of 41 magnum is 25 years.
The 44 Mag would be a close second followed by the 45 ACP. These are based on MY Favorite to shoot and load for not because I think they are the best rounds at what they do.