What kind of seeds? If you want just normal seeds, using a deployer/block breaker system that automatically plants seeds, bonemeals, and then harvests over a single block of tilled soil can get you a lot quickly and basically for free.
What are you going to do with the seeds?
If you're using them for seed oil, I would recommend using flax seeds as both the top and bottom portion of the plant can be broken for seeds. A automatic deployer/block breaker system can also be used for them.
If you need a lot of seed oil, I would recommend investing in tree breeding. Farming a couple walnut/chestnut trees will give you more seed oil than you know what to do with.
Edit: Whoops, ninja'd.
