Re: ♔ BUILDINGS Oh, my wrong. Isn't it just the 20% of all the gold spent since base plus upgrades… Just like every single building? I doubt developers would change that.
Best? Depends on how you define it. A level 4 guild with any attack building earns you the most gold for the price...but that's two buildings. For most gold in a single LL building on your first hit, that would be Titan's Lair. For most gold in a single LL building on an unload, that would be Circle of Elements. The strongest in attack is the Cursed Foundry. The strongest in spies is the Stronghold of Shadow.