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.
IMO, sticking with 1 clan is best, because you will always get the bonus....
for 2 clans:
Offense (damage)- Defence
or pure offense.....
Just remember though Nightmare depend on their life gaining ability's and stop opp ability crushes them , Uppers are a great choice one of the best defense bonuses and with cards like jackie and Dorian they can do great damage
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)
Yh every clan has some great cards and 2 clans is the easy option and possibly a Leader if u can afford one