A card, is usually a card with more than 5 Power, and either decent damage, or a good ability.
Clans are different, what makes a clan good, is the all around card count of usable ELO cards...
and the Clan bonus, and how effective it is.
Uppers for example are a clan with a very good bonus. And, enough cards with Power above 5 to be usable in ELO.
and Pussycats a very useful Bonus. with a lot of cards with Power above 5, usable Even at level 1... in addition to at max level.
the All Stars Clan is a good example of a Clan with Good cards, and a good bonus.
Cards like Striker, Alexei and Lamar make for great ELO cards, due to their Stop ability/stop bonus abilities, which are also very useful.
Uppers are good, because Zatman, a 3 star with a - attack bonus, in addition to a - attack ability.
Frankie Hi is good too, but has less power than Zatman.
but in general, Cards make a clan good, you can have a great bonus, but without the cards to back you up, then the clan won't necessarily be good for ELO.