There are quite a few good covens, but not all the covens that accept anyone are recommendable in my opinion. Spell Casters, for example, may be highly active(don't know, just an example), but the knowledge within that coven is about as trustworthy as my sister-in-law's boyfriend. For those of you who don't know, I would bet my life that the boyfriend mentioned has not told more then twenty truths in his entire life and most of those were probably because he couldn't think of a lie fast enough.
Anyways, back to the topic, I suggest you try out a few for real before judging them. Many knowledgeable members, myself included, avoid the Chatter for various reasons so other then what you see in the forums I could appear inactive by that judgment even though I log in and check my coven's forums twice daily. The private covens especially, the ones that you can't view the forums for unless you are a member, their activity is hidden until you actually join the coven and can see it for yourself. At which time, if you want to stay in the coven, you should stick around for at least two weeks and be active yourself. One reason some covens are inactive is that people go through a time of not being able to think of new threads and not having good replies to the posts already made, although they may still be checking the forums from time to time. New posts, in that case, would mean a new burst of activity and perhaps a break in the writer's block.
That's just a piece of my advice. I'd also suggest you start with a coven that has a topic you're into. So what are you wanting to learn?