Kaiserreich’s Left Kuomintang game-play is the gold standard against which all other HOI4 mods, demonstrably leftist or otherwise must be judged. It is engaging, reasonably ideologically coherent, uplifting(or not if you pursue one of the less savory paths), full of different choices that will impact the long term dynamics and where your new government ends up, tactically and strategically engaging with lots of different ways to engage in wars and alliances and perfectly utilizes the new HOI4 government power struggle mechanics.

With the recent updates, Kaiserreich has been getting better about representing leftist ideologies beyond the milquetoast reformist/wholesome “syndie”/red fash trichotomy it used to have, so I’ll still have hope, especially when the Commune of France gets its update after Germany’s (Syndicalist paths include some union-brained Marxists like Jack Reed or Oleksandr Shumskyi interestingly). For China in particular, there are certain Marxist paths for the Left KMT. To answer your question, not necessarily for “commie mods” but for those which have good commie paths…

Currently out or at least in stable alpha

Red Flood: Germany, Japan, Brazil, Iran, many in Russia, and so on. China seems to be just old or skeleton content but you can support Chinese and Korean revolutionaries as Red Japan.

TNO: Many Russian warlords from which you can reunify the USSR with and support non-playable commie revolutions around the world, plus some limited socialist/communist content for minor nations like Ukraine and Britain recently. There are some communist China-related submods like “East is Red” and “Ma Mod/Brave New World”.

Rivers of Blood: More arcadey and not really complete but it’s fun to tear stuff up as Rosa’s Germany.

Equestria at War: No, I’m not joking. It’s impressive just how well made this mod is. Lots of good communist paths which subscribes to the immortal doctrine of Caramel Marks, more recently like those in Zebrica.

Mods like New Ways and Rt56 introduce a lot of stuff for vanilla, though much of it is variable in quality

The many individual country mods for vanilla, even for Germany and Japan. Eight Years of Resistance is one that focuses on China and revamps a lot.

Some upcoming, in-dev mods made by actual marxists (though not necessarily ML)

Spartakus: Planned to release with Red Germany (duh), Reactionary France, and Civil War Italy. There’s a public playable alpha that’s a bit rough around the edges.

Reds!: Red America, USSR, South China (where the CPC-KMT United Front is maintained) and some socialist paths for France are planned for release.

Krasnacht: KR sequel mod under development. At release, America can go Marxist and West Germany is Marxist from the start.

I haven’t played a lot of Millennium Dawn, but Kaiserreich is probably at least three quarters of my HOI4 playtime. Keeping in mind that it’s all alt-history, and as such doesn’t have a lot of overlap with events in the real world, I enjoy playing the leftist nations in the mod. China is one of the more developed areas of the mod, but with an entirely different setup vs. IRL China. There’s a ton more content than vanilla HOI4, with almost every country having a unique focus tree, and there are events as far out as the 1950s for most major nations, as far as I’m aware.

I’ve also had fun with the USSR successor states in TNO, but I haven’t played much else in the mod. It was fun to rebuild the USSR from the remnants of the red army led by Zhukov.

There’s Red World Fan Fork, which is an alt-history mod where the USSR wins the cold war, but I haven’t had a chance to play it yet. And it’s not really communist specifically, but I’ve been messing around with North America Divided, which has communist paths for most nations (located entirely within North America, as the name implies lol).

Thousand Week Reich has several countries with communist paths, mostly USSR breakaway states. They are working on more for future updates which look interesting.

Another interesting one is Pax Britannica. Its fairly new with not a lot of content yet but it has a fun path for the USA with Smedley Butler as a Mao sort of character complete with his own Long March. This mod has “Jacobinism” as the communist equivalent ideology from my understanding.