There is no such thing as the best coven. They are all different and each has its share of positive attributes. You will find certain covens work better for you simply based on what you're looking for.
The choice comes down to you and what you feel would be the best choice for you specifically. Anyone can tell you which coven they think is better than the others, but just because that coven works for them doesn't mean it would work for you.
Instead of asking us what we think is a good coven, why not take a step back and make a list of what you think makes a good coven and cross check it with what the covens here offer. Do they offer what interests you? Are they active? Do they offer classes to help anyone new to magic? These are just a few examples of things you might want to ask yourself when looking for a coven.