There is no best. My recommendation is to download a bunch, then uninstall them one by one until you end up with the one that you like best. I settled on my favorite, and I had to give up some great apps with great features to get there. What do you value in a forum-style app?
That's what I use. Didn't want to influence your decision, but there's no harm in revealing it after the fact. I do feel like I missed out on some cool features from the other apps I tried, but Eternity was the complete package.