I actually like the circular one, but the font that you've used doesn't fit with it. It needs a much more organic typeface to continue overall, organic-ness of that image. I think that one could be the strongest, with some more work. 😉
In all honesty, the three on the bottom right are the worst of the bunch, so I wouldn't use those.
I didn't have the time to illustrate before what I meant by "organic" so I threw an example into this post, which may help you. Since I took yours and messed with it, the file quality of the attached is an absolute mess, but it's only for illustration purposes. My point here is though that you should try some organic fonts with this as they look better than the one you've used. Arial? I can't really tell which one it is. You're doing the typical rookie thing and sticking to one font that you think is safe, rather than experimenting. 😉
I used one here called Quango which is free for personal use, so maybe not suitable for your forum. However, there are plenty of fonts out there, so experiment. Try Font Squirrel. If I need a font for a particular project and the client isn't going to pay for it, I go there. All of their fonts are free for commercial use and of professional quality.
Maybe even make the "A" slightly smaller so that it still spills over the green circle, but not quite as far as it is atm. It'll take up less space in your header that way, IF you are using it.