Most of us didn't start out being leftists. Lots of us have lib or chud backgrounds but was radicalised by material circumstances and leftist propaganda.
We want more people to turn left and we cannot do anything to change the material conditions of capitalism. We can however work to improve our propaganda so it will have a broader appeal.
To get a discussion going about what is good propaganda we could use our own radicalisation stories as a starting point.
I have always been a weird nerd so for me it was listening to long lectures on YouTube that helped move me from being a succdem to believing that capitalism has to be abolished. Around the time of the financial crash in 2008 Richard Wolff's Capitalism Hits The Fan as well as David Harvey's lectures on Marx was hugely influential in pointing out how capitalism is foundationally incompatible with human progress. Talks by Chomsky helped me realising how thoroughly evil US imperialismnwas and Parenti's content made me question the Cold War propaganda about the USSR and its allies that we are indoctrinated to see as common sense and get a much more nuanced view on things.
Unlike the postmodern neomarxist utopias that chuds think universities are, I went through a full philosophy degree without a single mention of Marx/Marxist theory -- despite minoring in political philosophy. The amount of time spent on the politics of Hobbes, Smith/Hume, Kant, and Nietszche (not to mention Nozick, Hyek, and Friedman -- albeit critically), made it clear to me that, despite the study of philosophy being broadly about learning to think about how things tie together in the broadest ways and Marxist theory a concrete societal analysis of that sort, the academic institution and its deference to hegemonic 'classics' will continue to function to obscure and redirect the critical abilities that it itself works to produce.
Obviously my experience is but one, and I'm sure there are some based philosophy profs out there. I just think that on the whole, a stress on rational reflection alone breeds only liberalism (at least when the student/professor body is encapsulated by a capitalist/imperialist mythos). That's not to say that an education in philosophy is useless. For me, the most effective propaganda for radicalization was stuff that both engaged in 'careful' intellectual reflection and appealed to empathy and the subliminal sense of mass injustice that I didn't have an answer for (and which would've been attributed to 'religion bad', 'orange-man bad', 'conservatives bad', or whatever other myopic and intentionally isolated cause would be sufficient for a local explanation). To that end, I went from Sam Harris' thin veneer of intellectually honest politically engagement (and his favorite 'orange man bad spiel), to a brief spell with Destiny's more abrasive (seemingly) rigorous political debates (fuck nazi's amirite?), to Vaush who at the time had a small channel and came off as much more empathetic to oppressed peoples while also still dunking on nazis. From there, he had a brief time where he was listening to the chapo podcast and complaining about how people in the chapo subreddit hated him. I checked out the subreddit despite what was basically a condemnation of it from Vaush, specifically because of my background in philosophy. I really do think that without that drive for self-critique, I could've easily stayed at the start of that liberal-conservative pipeline (or worse... at one point i remember listening to half a Molyneux podcast... couldn't get through that even as a political illiterate though, lmao)
I know that that's a bit of a ramble. In terms of the most effective propaganda for radicalization, I would say that throughout my radicalization process, capitalist propaganda itself was the strongest tinder for radicalization, granted that (1) the subject is shown at least once that it cannot be trusted or taken at face-value, and (2) that once such propaganda is no longer taken at face-value, at least some of the internal contradictions are clearly laid bare. Some of Vaush's and Hasan's videos are good for that, just as most chapo posts are. (The main problem with Vaush, from a radicalization perspective, being that he's a grifter who actively attempts to isolate his community from political thinkers that are to the left of him, partially out of motive for status and financial gain, but partially because he is a lazy thinker who has and will continue to benefit from uncritically towing the imperial line.)