Mumsnet. Rich named this community his favourite back in 2010 and it's still going strong. Back then it was highly active, highly ambitious, highly important to members, and the same can still be said. The site is easy to navigate and I particularly like the magazine style homepage – it doesn't present a list of categories from which to choose, but rather offers a variety of interesting information, encouraging readers to dig deeper.
Threadless. Another community that has stood the test of time – they kicked off in 2010. Threadless built their business around the community rather than the other way around, and they have managed to nurture that relationship and go from strength to strength. I also love the fresh, modern and intuitive design.
Figment. This example is interesting in that it targets a very small niche community, part of the larger Random House brand. Figment is a community for young readers and writers of fan fiction. This laser focus means that members are highly engaged, and with more than 300,000 members creating and moderating content, brand awareness and advocacy are high.
Adobe Learning Communities. Another example of an incredibly strong branded community (or in this case, set of communities). While the UX isn't great in terms of accessibility, these communities boast amazing engagement rates and response times, and afford members the opportunity to influence future products. Adobe's community team works really hard to make sure that every member, regardless of expertise level, receives appropriate help, support and challenges.
SitePoint. If we're talking longevity, SitePoint takes the cake. First launched in 1998, this community was one of the earliest tech communities around and has overcome many challenges and several changes of the guard but is still going strong. SitePoint was one of the first (and definitely largest) migrations onto the Discourse platform, after 15 years on vBulletin.
SAP Community Network This is an interesting one. The community is huge, diverse, and highly engaged, which is a big achievement for a branded community. The design isn't great and the homepage is a bit overwhelming, but it seems to work – SCN boasts over 2.5 million engaged members!
Money Saving Expert MSE is a highly engaged community that consistently pulls in the numbers (37k online at one stage in 2015). The UX isn't great, with small, low contrast typography and a mind-boggling number of categories from which new members have to choose, but they must be doing something right to keep consistently high engagement.
My Starbucks Idea. Here is a fantastic example of a successful ideation community. Rather than a community to talk about Starbucks, ideation is used as a hook. The design won't set the world on fire, but retention is maintained by actively listening to feedback and following through with the consistent implementation of community suggestions.
[topcoder] This is my personal favourite. This competitive coding community has a UI which is clean, fun and intuitive. Gamification principles are successfully leveraged to stimulate a high level of sustained engagement, while still attracting a professional audience. With almost a million active members, [topcoder] brings together designers, developers and data scientists to collaborate on paid projects.
Ravelry wins the prize for the cutest splash page. Add to that the fantastic on-boarding experience, the built-in collaboration functionality (allowing members to invite new members to join a project), and smart integration of e-commerce, and this community for crafters is a shining example of a successful online community. Of their ~6 million registered members, over a million are currently active.