The best for causual games are probably the Hoax and Vulca. Thats why they are the base set. The Mimix and Terrah are also farily straight forward. OTOH if you want more of a CCG feeling get the Flit and the Khind. Those two have the most special texts.
I think that the Aqua are probably more straight-forward than the Terrah, and they have a different feel to how to play them, which is nice. The Terrah and the Vulca feel somewhat similar to play.