You get into the definitions of good and evil (as defined in D&D good is more or less utilitarianism, evil more or less Social Darwinism from what I can see), which is of course going to be controversial.
As far as modrons as big bads, the Borg is one good idea. If you wanted to be really nasty they could start developing immunities to every type of attack the party throws at them enough times--that was one of the big problems with fighting the Borg if you remember. If they're new arrivals to the plane, they start out with no immunities. The party throws fireballs at them, they start developing advantage against saves to that, then resistance, then immunity. Of course you're going to have to find some way to limit it or they'll be immune to everything.
A more simple tack is the invading 'civilizing' empire, which of course wants to subjugate all other populations as part of increasing the amount of order. Real-life colonialism, the Soviet empire, etc.
There's the whole Hellraiser 'pain is orderly' argument, which I always found more Lawful Evil.
Organization makes you more effective (it's why they had to postulate a larger demon vs devil population in the Blood War), so they can be pretty effective opponents.