I don't think level 10 seeds exist. Even some of the toughest plants to garden, such as Queen Myrtle, only requires Rank 7 in gardening (even though the spells to tend to her pests are Rank 10-15).
As for what seeds you should plant, it depends on what you're going after, but pet snacks is usually the answer to that question. You should plant Couch Potatoes, which you can find by defeating just about any enemy in Grizzleheim. They drop some of the best pet snacks in the game. Evil Magma Peas drop from enemies in Avalon and also drop great pet snacks. Once you reach Mirage, it becomes even easier to farm for those seeds over there.
If you're after Amber to craft spells, then King Parsley is the way to go. There are a lot of enemies that can drop these, but my favorite way of getting them is to do Master or Exalted duels and get an Ultra King Parsley, then plant that and get some of the regular versions that way. Not to mention that Ultra King Parsley is the best plant for obtaining Amber, constantly dropping it as part of its Mature harvests.
It also helps to check out the Wizard101 wiki if you're curious about what certain plants do. There are many different things that can be provided by a variety of plants, it just depends on what you are after.
