Which you should strive for depends on which appearances you like. No one else can answer that, but there are two easy ways for you to look through them and make those choices. You can either visit The Dreamgrove, equip the appropriate artifact, and click on the Seed of Ages, or you can visit this link for Wowhead’s tool.
The second row from the bottom (Might of the Grizzlemaw/werebear for guardian, Ghost of the Pridemother/sparkle kitty for feral) cannot be obtained unless one beat the appropriate challenge back before the Mage Tower was removed. All of the other rows can still be collected.
For my own personal preferences, I rather like the Moonspirit for feral and first, third, and bottom rows for guardian. A big, neon blue laser bear (Avatar of Ursol) is pretty hard to miss!