Any good suggestions on one that is a good mix of casual and serious players that play together often?
CORP (despite the name, the roleplay part is completely optional) is a large clan with lots of active and friendly players. Some of them play seriously, some of them (including me) a bit less.
You'll always find people who'd like to play. No matter what type of game you propose, someone will always be ready for it.
We also have cool forums & chats if you're interested.
If what I just said meets your expectations, you can send me a mail to further discuss the possibility of joining :)