Well it all depends on thier bonus's. You get a clan bonus for having 2 of them in play on the field so ideally you want a deck made up of 4 cards from 2 different clans. With that in mind choose 2 clans that fit your play style.
Clans that synce up in that one is a blocking clan, another is an offensive clan. Generally.
Although you cant go wrong with this deck.. Which is lame tbh.
The truth is using 2 clans is the best because you can put the 4 best cards from each clan in a deck.
There are absolutely no clans with 8 really good cards
And even if you use 2 clans, there is like 80% chance you'll get your bonus anyway.. (and if you don't get the bonus, you still end up with a very good card)