Got more than 1k games with all kinds of Skellige decks on my back and i can tell you Devivre's advice is pretty solid. One thing you will notice soon enough, if you want to make a competitive Skellige deck, is you got very little choice on silvers. Sigrfrida, Restore, Decoy and Donar An Hindar are autoinclude in 95% of the decks, Udalryk not so much, but comes close. Ermion is a pretty good gold and you will need either him or The Last Wish(or both) in some discard heavy decks, but adding him on other kind of decks it's completely up to you and the other golds you could need as a core. Birna Bran is autoinclude in every Skellige deck, period. About bronzes, i have yet to find a deck that doesn't work better with 2-3(generally 3) Priestess of Freya and Clan Dimun Pirates are pretty much a must for every deck too.
Clan An Craite Raiders, War Longships and Queenguards are really good for discard-heavy decks.
Berserkers, Savage Bears and Clan Tuirseach Axemen are a must in self-wounding decks.
Clan Tuiseach Skirmishers are good cards which fit in a bunch of different decks, but they're simply outclassed by Light Longships in the current meta.
Clan Heymaey Skalds and Clan Tordarroch Shieldsmiths are really good cards overall and could serve as filler in almost any deck, and even as core in the case of Shieldsmiths.
Overall, just stay away from Clan Brokvar Archers(these ones could fit some decks, but decks are just too small and Skellige has way too many autoinclude cards currently) and Hunters, Clan Dimun Pirate Captains(this could change after incoming weather update tho) and raging Bears(to devs: give them some love PLEASE), and you'll be fine.
As for leaders, Harald is the best one hands down, but Crach can be better in some decks since it can put a lot of pressure early in the game and that's something Skellige definitely lacks overall. King Bran is simply the worst Leader in the game in my opinion, the only way you can make his ability any useful is by using it with only 1 card in hand and even then you will waste the base strength bonus it provides unless you're discarding a creature(still wasting 1 of the bonuses) to get a way to resurrect her.
Sorry for the wall of text, i freaking love Skellige.