In Garry's Mod, there's a fan-made addon called
Vindictus Boss NPCs. Basically, this addon lets you kill these giant mofos from the eponymous MMORPG.
And more. There's
a lot of them to play with.
Now, since it's Garry's Mod the ultimate sandbox you can easily cheese them with all the powerful tools in the game such as Ness's Homerun Bat, the Infinity Gauntlet or any sort of admin god weapon. You can give yourself god mode, yeah, or a lot of HP and shields. Since their AI is constrained within Source's limitations, you can also cheese by sniping them from far away.
But if you get close enough to them to be "aware" of you and you don't have any sort of cheats or cheap weapons on? It's hell. From their god-like amount of HP (in the range of 10,000 HP and more, the stronger NPCs that come with Garry's Mod like the Antlion Guard are only in the 100's.) to their powerful large AOE attacks, they take a lot to go down, and dish out lots of pain. And there's even a setting in the SNPC addon that lets you crank up the difficulty even more, and it's already pretty tough, even on normal. I typically use the Call of Duty World at War SWEPs to kill these but even then they're actually on the stronger end of the weapons than say, the default Half-Life 2 weapons you start out with.
In terms of other games with dedicated bosses, I haven't played Cuphead or Dark Souls but man, I'm not going to have the patience to beat those bosses after the hell the protagonists from those games are put through and what I've seen from them.
Of course, there's also the Super Smash Bros. bosses like Master Core but honestly, the hardest part about them is reaching the *bleep*er. Not that reaching it is hard. It's just that i have no patience to sit through the rest of the single player game just to challenge Master Core.
Team Kirby said:
4. King Dedede (Kirby Air Ride)
That *bleep*ing timer is more of a dangerous threat than King Dedede himself. At the very least, in City Trial if the Vs King Dedede Event is the one active, that event spawns much more beneficial items than normal in the City Trial run and you're more likely to come across random All-Up patches here, which is very very neat.
Only two Copy Abilities are worth a damn here: Tornado and Spikes. The rest do jack in terms of damage to King Dedede, and you always deal the most pain when his terrible vehicle jams himself in a corner.