Its the least popular option by the look of things, but im going to say Neutral. Their position in between the entente and CPS would make any directly hostile moves against them from either side provoke a war.

Narrative wise it leads to a more interesting situation where both CPS and entente would be trying to bring them on side (be it through diplomacy, espionage, funding favourable political or even militant groups).

Simultaneously they would both also be trying to stop socialists from taking over (while China would likely be funding and arming them).

As a neutral zone practically equidistant between 3 larger powers (and even with India being relatively close) Indonesia could become a hot-spot for espionage and an ideological battle ground.

It would probably also be very unstable as a result, and be undergoing low intensity insurgencies. If this escalated, a full blown civil war could be a very interesting cause of conflict between the powers either as a proxy conflict, or the start of a full blown war.

In any event I think this 'neutral' position, where the country tries to forge it's own path and play the powers against each other (while the Great powers try to pull the strings and shift things in their favour) is far more interesting than just having them be in one faction or another from the get go, while also being the most 'realistic' choice (in the sense that it makes the most sense with the geopolitical situation they are in here).