Treatment for fluorosis of teeths best painless longer lasting method?

Veneers best but... The amount of dicoloration will determine the best esthetic treatment. Tooth bleaching will give satisfactory results rarely for this condition. Tooth bonding can help most people the result is very dependent on the skill of the dentist. Bonding is painless, lasts if done skillfully, and can be done without damaging the treated teeth. Veneers last longest, but your teeth are damaged.
Fluorosis. I am assuming you mean the esthetic treatment to mask the discoloration. Besides lowering the amount of Fluoride you are exposed to and treating any weakened tooth, there are multiple options depending on severity and location. Bonding, veneers or if needed even crowns if there is a lot of damaged tooth. Even bleaching may help blend. I would see a trained cosmetic dentist for your options.