I'd say BL for sure for several reasons - sadly you can't sign up for this season any more iirc.
If it's not BL, I'd actually say join an event rather than anything you listed above. Guilds are dead. War is a huge investment of effort and time, if you have never participated in anything PvP related, you might be overwhelmed. Brawl is good for the social aspect, but it doesn't increase your skill with anything ingame related... (You could say it does if you apply as a deckbuilder, but who bids for an unexperienced deckbuilder who wants to learn how to build decks by participating in Brawl? :p)