Which social media channel do you find most effective to build community?

Bear in mind that building an audience is different than community. For both, my preference is Twitter. I find it easier to build community on Twitter than other social sites. LinkedIn can be good too, but it's harder to do it from a community perspective, having a process to consistently connect with a specific set of people is hard. Yes they have groups, but they just get bombarded with sales spam. In contrast, Twitter you can do Spaces, have Twitter Communities and also gather around a hashtag. This leans more towards community building, rather than audience building.

Twitter.

Twitter - hands down. LinkedIn is good if you're a company and want to engage and inform your community. But if you want to build it, then Twitter's the one you really need to leverage (but it does depend in which sector you find yourself in, if you're trying to build a community for legal people then Twitter might not be your best bet...).

LinkedIn is the best social media channel; I use LinkedIn to build a community in professional people.

It depends on your goals; for me, LinkedIn is the most effective social media channel to build a community of professionals.