The best diplomacy system i found was in star ruler 2 with the card system, you could do lots of nifty things and gaining leverage against other empire was a thing to force a vote your way, IE: The enemy has a colony in your system, well pss a vote so they turn it to you, and if it happens you have allies who hates him they might even pitch in your favor!
While the system itself was kinda cool, it fell short for me mainly because it well...wasn't a diplomacy system?
I mean, it didn't actually change how other empires think of you in the slightest, it was all action and no reaction. The AI never got mad at you for taking subversive political action against them, nor did they have any reaction to more neutral/positive votes.
The ACTUAL diplomacy was all done in the same nigh-random tick box dropdown that plagued SR1. Which is to say, was entirely vestigial and likely went completely unused.
The entirety of it's ACTUAL diplomacy system could be boiled down to "are we at war? yes/no" and "are we allied/a protectorate yes/no"
And that is fucking HORRENDOUS.
I mean, I suppose it really only shines in multiplayer, but frankly when you're playing against several other real people you don't actually NEED a diplomacy system, as you all can figure out where you stand on your own.
So yes, congats to Umbra and their team for creating a wonderful system to simulate political fuckery, but DON'T YOU FUCKING DARE claim it's better then other diplomacy systems, don't claim it's better then systems with AI that actually react to what you do and have goals beyond "kill everything not me" and CERTAINLY don't put down those "utterly stagnant, banal" systems, because they ACTUALLY FUCKING SIMULATE RELATIONS BETWEEN FACTIONS, which SR2's system...does not.