In that framework, the King/Emp has a plurality of power (IE the most powerful) but he doesn't have the majority of all power. Many other forces could team up to take him down, so he has to constantly work to keep them from doing that. (Either by appeasing them, or pitting them against each other. In the film, we see both (Appeasement and conflict)--but only the pitting of the landsraad against itself is shown overtly. In reality though he's also APPEASING The Guild.)
And the exception to the above 'he's probably the strongest' is probably The Guild--overall its power is probably more than that of the Emperor. But its soft power. You could kind of think of it like the Catholic Church, or OPEC or like the banking industry of the reinesance. They don't have any hard power (Troops/military) but they control the economy, so really they can get any source of hard power on their side by promising tons of benefits. In Dune specifically, they are the only ones that can get people between planets--so if they cut off your systems, your economy is fucked. If they choose not to transport your troops, your military is fucked. However, they have no real troops and can't actually go down and force people to do things (IE no hard power).
So the Emperor is their go-to guy for this, and a big concern of his is ensuring the Guild never looks elsewhere for that kind of 'hard power'. (And this sprawls into a huge behind the scenes political framework, where you eventually see the guild didn't keep armies intentionally so they'd be overlooked as a source of direct power and be free to manipulate more from the shadows. Mainly because all they care about is ensuring spice production and don't give a shit about anything else.)