In addition to the suggestions above, make sure the scale is on a non-flex (hard) surface.
I have 2 scales. My Tanita measures to 300 pounds in .5 pound increments with a body fat percentage available. It has been very accurate. I have a new Taylor scale that measures to 440 pounds in .2 pound increments with a BMI option. It has an oversized platform to stand on. It is also quite accurate so far.
It really depends on how much you want to spend. The range varies by weight above 300 pounds, if you can get bmi, body fat percent, or bio impedence analysis (BIA), if it allows tare (zero out) option. I found scales from 29.99 to 1600.00 usd.
I am not sure what is truly the most accurate but, I have read reports indicating detecto, seca, and health-o-meter are among the best of the medical scales. Tanita, Weight Watcher, Taylor, and Omron all have been well reviewed.
I found the article in the link below to be informative for me. Maybe you will enjoy it too. They used the reviews done by consumer reports and good housekeeping as a basis of the article and explain why the excludes other consumer reviews.