The best containers for sowing seed are those made for that purpose!  I have found plastic to be preferable to molded peat products because plastic is cleaner, doesn't disintegrate so is reusable, and is less liable to carry disease. Some seeds do lend themselves to being started in individual plugs, especially those seeds which are very tiny.  If you need to start your seed in a community tray and then transfer to separate pots to grow on before planting in the garden, a plastic or metal tray from 6" X 9" to 12" X 12" is appropriate, as long as it is about 2 ¹/_²" to 3" deep so that the potting mix can be the optimal depth.  The trays should be filled to the top and the potting mix leveled off even with the top of the tray.  This will encourage good air circulation and discourage disease.  Drainage holes are important so you can bottom water and also so the planting medium doesn't get too wet, which would also encourage disease.

Many gardeners still like to use molded peat containers, which do eventually disintegrate and so they meet biodegradability concerns.  Peat pots or plugs must be totally buried when planted in the garden. Otherwise, they will wick moisture away from the seedlings, drying out the roots. Larger, individual pots (2 ¹/₂ - 3 inches) lend themselves to growing larger seed or a plant species that resists transplanting, for the seed can be planted in the peat pot, grown until ready to plant outside, and then planted pot and all directly in the garden.  When using peat pots (such as Jiffy products that revolutionized seed sowing 50 years ago), you will probably need to place them in a plastic or metal tray for support and for bottom watering.  One caveat: do not reuse peat pots or trays; the pennies you save will be more than offset by the likelihood of fungus growth.

And, of course there are many make-shift containers that are usually part of any household miscellany - paper cups, baking trays, plastic food containers, wooden boxes you make yourself - that can be used to hold planting medium to start your seed.

Whatever growing container you choose, there are some requirements that need to be met for successfully starting your seed.

1) The planting medium needs to be 2 1/2” deep, so the sides of your container need to be at least 2 1/2”, but not so tall they block air or light. Up to 3” deep is fine. 2) Good drainage is essential. If your container doesn’t have holes in the bottom, punch some in. You can then bottom water to keep your seedlings evenly moist, but not soggy, by placing your container(s) in a tray that will hold water.

2) Good drainage is essential.  If your container doesn't have holes in the bottom, punch some in.  You can then bottom water to keep your seedlings evenly moist, but not soggy, by placing your container(s) in a tray that will hold water.