Which Should I Do First, Braces or Veneers?

If teeth are misaligned,or too far erupted then it is best to move them into proper arch form before veneers are done. This will result in less tooth structure being removed when veneers are prepared.

The teeth should be aligned first before placing your veneers. Aligning the teeth first will minimize the amount of tooth structure removed before placing the veneers.

The ideal treatment is to do braces/Invisalign in phase I to get your teeth positioned so that I can be the most conservative with my porcelain. Then move to phase II - porcelain veneers.

The advantage of braces before veneers is the position and angulation of the teeth can be corrected and idealized. Be sure to let the orthodontist know that you plan on veneers at the end, as that may change some approaches to treatment.
Well placed teeth allow the veneers to be as conservative as possible. The veneers will correct the color and shape of the teeth.
If your teeth are mostly well positioned, then the braces would be completely unnecessary. Minor imperfections can be corrected with porcelain veneers, which is why porcelain veneers are sometimes called "Instant Orthodontics".