Well packaged seed with a label that conforms to federal law
Get to know the company. Outlandish claims like 100% feminization, 100% germination, extremely high or low cannabinoid content should be validated with respectable 3rd party data.
Proof of stability: Stability means the variety has performed consistently for a number of generations. Asking for data on the parent generation can be helpful in determining of the genetics are stable. This is particularly important for THC content. Well-bred seed should have stable THC content across at least 2-3 prior generations
Look for AOSCA certified seed: This is 3rd party data that certifies that the particular seed has been grown according to AOSCA Standards to maintain genetic purity.  A seed certifying agency works closely with seed growers to help them follow AOSCA Standards throughout the seed production process.  AOSCA Standards apply to hemp and a wide range of field crops, turf grasses, fruits, vegetables, woody plants, forbs and vegetative propagated species available for sale. Seed lots that successfully complete the seed certification process qualify for the official “Blue” Certified seed tag, providing assurance to the seed customer that the seed has met standards for genetic and varietal purity.
Look for seed that suits your production methods and equipment.
Look for seed that has demonstrated performance in your region, or has been bred in your region using data based breeding.
Buy seed once you understand the market that particular variety serves. Identify customers for your crop before you plant.