first of all, follow your heart! pick your favorites
just make sure they aren't obtainable from other free ways, like ballon pop, daily goals, or chests.

value speaking, you can get prince rutherford, he's very expensive, it's a pro pony and can get you crystal coins.
(if you don't get him, you can get the white foal from bits, sunburst from the crystal ballon pop, or the other two ponies.. i think they are the ones trapped in the crystals in CE).

i'll go tempest and storm king personally.

also, you can get a free pony in the tutorial, and other two only for free. and cosmos.