Twilight Forest has a few boss mobs, but they aren't that hard depending on the mods you have installed.What are the popular boss mob mods right now? I want something rather hard and with good rewards.
Lycanites mobs will always be a real b****** but they aren't really "boss mobs"
There's Wydlicraft (probably spelt that wrong) which adds very powerful bosses, and very powerful tools. But they are still pretty tough.
Other than that I feel boss mobs are lacking majorly in modded minecraft... expect JordsUtils to have a boss mob or two at some point