I would use Crest Whitestrips Supreme. They're the strongest, but they're only available through the dentist. After using it once a day (30 min) for 7 days, my teeth grew 3-4 shades whiter.
Also another tip, don't brush your teeth before bleaching, and make sure you use a tissue to blot off excess moisture from your teeth before you stick the strips on. That way, your teeth won't get damaged and you'll get maximum results =)