1. CK2 - Most polished of the titles and unlike the other Pdox titles I've played it doesn't have overtly broken mechanics or reliably lie to you every game. Plus it has the best stories and a lot of flexibility.
2a. EU 4 - I have the most hours in this because I played it first. Recent patch trends have been incoherent, troubling, and continuing the trend of telling the community "we care more about esoteric exploits than that the game has lied to you in multiple proven/documented ways the past 3+ years". Bonus points for showing idea group data and then nerfing average-to-below idea groups while leaving some in the top 5 untouched. Double bonus points for the broken corruption model + stating that supposedly players needed incentive to use TCs, when players were previously moving their capital to use TCs for years.
2b. HOI 4 - Bad as some of the trends in EU 4 are, aside from corruption/religious conversions the core mechanics still work. Such is objectively false in HOI 4. Occupations and peace conferences are both designed poorly *and* carry objective bugs. War contribution is a complete joke. Similar to EU 4, the game lies to the player...this time in both achievement descriptions (at least five of them outright lie about requirements) and in national focuses (can create faction =/= can create faction). Since some of these are so ruinous they force player to either restart runs or spend longer in current run than restarting would take, it gets downgraded past EU 4 still. Bonus points for marking a bug report as "resolved", despite that doing so necessarily concludes that Curacao isn't in the Caribbean. More bonus points for recently stating incoherent logic as a basis for "artificially extending a particular war arbitrarily"
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So yeah. CK2 is pretty good. The rest have shown either steady decline or never functioned properly to begin with. You could make a case for moving EU 4 lower though since it did falsely advertise cross platform MP.
I've not played enough Vicky 2 to have an opinion. On principle and lack of trust, I will not buy Stellaris or Imperator. I don't consider their odds of being good especially high given the design philosophy stated for them. However, this does mean I'm ignorant of those three for differing reasons and they might be at least average-to-good, so I won't place them in rankings.