This is a tough one for me. I can't say Orange, because every season they start hitting their stride right about this point. I don't want to say Stars, because they got their bid, but still struggle putting 2 great performances back to back. Same thing with F5. Many of the others mentioned in this thread have been good all season. A bad performance here and there doesn't take anything away from them.
So, with that all being said, I would probably say the most improved team so far this year is CEA sm sr. There are so many teams that have improved, so obviously you can't go wrong naming any of them. I will say the team that has shocked me the most is WC Suns. They struggled a little at their showcase, but that's been it. They've been great everytime out so far this year.