My personal favorite is Sewari, a park located between the Kizu and Yodo rivers halfway between Kyoto and Osaka cities. There are over 250 sakura trees lined along both sides of a walkway that leads to the end of the peninsula between the rivers.
In the past few years, either because I and so many others have posted pictures and promoted the place, Sewari has become more and more popular. During sakura season, thousands of people sit along the grass beside the trees along the river to enjoy the cherry trees, play games and spend the day.
It is a wonderful place to spend the day during cherry blossom season!