Really depends imho. Nevertheless, the nightmares do provide relatively cheap utility cards compared to the other clans - artus, mawpin, dieter, bill.
Skeelz? They have a wide variety of abilities and function at near full functionality even with a deactivated bonus.
Vortex? Their bonus doesn't affect either their ability to win a fight or their damage output, and they have several topend characters.
PCats? They have one of the best 5* characters without bonus (Lena). In Classic they also have Yayoi who goes toe to toe even if she's bonusless, and a top 20 2* (Lucy).
Junkz? They have lots of some of the best abilities (SOA, pill manipulation, power/ attack bonus, DR), 8 power characters with a combat bonus, a top 5 2* (Gil), two top 20 3* (Tremorh and Jiro), a top 20 4* (Qubik), and the best 5* in the game (DJ Korr Cr, and indeed a feast of other 5* characters to choose from (Peeler, Taham, Fuzz).
Honestly, the answer is "all of them."
But, it's how you mix clans that really counts. When you are making a dual-clan deck, the best strategy is to pick cards that can survive without their bonus, and compliment each other in some way. Your typical hand should have strong offensive cards, but offer some protection from the common strategies seen in the mode you intend to play as well.
A simple example: Putting Freaks and Jungo together works well. Jungo life gain works well with Freaks poison -- you can think of life gain as lowering a poison's min. (In case you don't get the concept right away: If you are playing ELO and hit with Olga in turn 1 and hit with Rodney in turn 2, you do a total of 13 gap -- 11 from regular damage and poison, and 2 from Rodney's life gain. On the other hand, if you hit in turn 2 with a 5 damage card, you only do 11 gap total, since the damage caps out at min 3.) Furthermore, both clans have high natural power and attack/power manipulation abilities, which makes a Freaks/Jungo deck suitable for winning 3 rounds, even if you draw a card with no bonus. Even better: Jungo have 5*s with decent damage AND round winning abilities, namely, Askai and Ongh... Including these cards will help your deck remain solid vs SoB, which will rob you of a lot of damage.
I think it really doesn't matter, It just matters what your playing style is.
Skeelz it just doesn't work for me because theres no 2HKO
Nightmare Is the same thing, just to hard to work in the KO'S
Fang Pi Clan Kind of like it , they depend on there bonus too much
La Junta I love because they have lot of cards to choose from Like Gatline, Dacote, etc
Pussycats Is for strategy, + life and - damage kill u
Rescue doesn't work for half deck
Sentinels are pretty good half of the time, Ghiest Roots Nightmare and Piranahs basically kill
Nightmare Has a wide range of strategy
Piranahs Pill manipulators that kill you pretty fast
Ghiest Pretty good most teams dont have too many stop cards ( for half deck )
Jungo very hard to beat, basically like a +2 damage but they can't KO you with it
Freaks a little obvious
Ulu watu don't use it too hard to play in half
Junkz BEST FOR A HALF DECK or Piranah;s but they need there ability to survive Junkz don't
It has Neil, Taham , Qubik, Tremorh, Fuzz , Sterik
Basically a piranhas pill manipulation combined with attack manipulation
Very hard to beat for a half or mono