I think there shouldn't be Boss Monster's. The idea of a Boss Monster means I'm playing a video game. I prefer to present my players with a living world. If I wanted them to play a video game, I would tell them to go do that.
Er...the idea of boss monsters long pre-dates video games.
No matter what you're fighting - be it a village of Orcs, a guild of Assassins, unknown horrors in the forest, or whatever - there's almost always something behind it all, and because might makes right often that 'something' either is the biggest baddest thing out there or has surrounded itself with them: the Grand Leader of the Orc village, the head of the Assassins' guild, the Lovecraftian thing corrupting the forest, etc.
And in the end, one way or another the goal of the party (whether they know it or not) is to bring that key individual down.
An interesting twist that IMO we don't see often enough is to have the truly toughest foe in an adventure not be the 'leader' at all but instead be a major pet or bodyguard or similar. For example, the party might be out to knock off some raiding Orcs and have backtracked them to their village. Scouting reveals the village is held together and run by a Grand Leader to whom all the other Orcs bend the knee, so that's who the PCs key on as the main foe. What they don't know is that the Grand Leader herself is a pushover, and the real threat is the Ogre she keeps hidden in her longhouse.
Or, another take might be that while the Grand Leader herself is a pushover the rest of the Orcs will die to protect her, making the "boss battle" a fight against the whole village.