Based on my research there are really only 2 viable poker forums that are not very small and niche.
CardsChat is a great forum for members learning how to play and who want to take their game to the next level. You will be treated with more respect at CardsChat. There are some valuable tools and information to help you become a better player.
CardsChat offers tons of events for their members and can fulfill the sense of belonging that many players look for. There are leagues,
freerolls and fun special events to participate in.
It is also a very global community - with players from all over the world to share poker and even personal experiences with. There are a lot of long-term and loyal members at CardsChat.
2p2 is the biggest
poker forum - but too big for many beginner and intermediate players. And their reputation for members being harsh doesn't make it welcoming for new members. They were so hugely successful back during the boom they will always do well and be at the top. They offer no events for their members.
Many mid and high stakes players are able to find more of what they need there, and there is a lot of valuable content there.
They also have a lot of long-term loyal members.
Both are great poker forums that serve to meet different needs of poker players.