Tapatalk could stand for improvement in many areas, but it works pretty nicely. It's been updated a mind-numbing 36 times since it came out in 2009, so the developers take maintaining it and enhancing it seriously. I think it was well worth the $2.99....
One limitation to be aware of is that some of these forum apps are supported via plugin (e.g. in vB), so your forum will have to be compatible with the app in question. These are a couple of the competitors....