Depending on where you  live, the classic big round ball hydrangea (Hydrangea macrophylla) with the brand name "Endless Summer" hydrangea gets my vote for the easiest to grow. This is because the plant will flower on both old and new wood so if you have a cold spring you will not lose the flower buds to frost. Another easy to grow hydrangea is Hydrangea 'Invincible Spirit" and this is a mountain type of hydrangea (Hydrangea serrata] with a bit smaller pink blooms but it is very hardy even if you live where the winters are harsh.  When it comes to hydrangeas you get what you pay for and the newer varieties really are improved but cost more than the old standby hydrangeas. Keep growing, Marianne Binetti