I’ve dabbled around with a few drawing applications over the past few years. This is my own subjective opinion, and the situation around each software is very likely to change down the road from here (August 2016)!
Painttool SAI -
It was my first drawing software back in 2010 and I do still love it a lot. I no longer use it much though - the lack of filters and text input can be quite frustrating. However it’s still a really solid software to start with and tons of amazing artwork get produced on it (iirc the top chinese artists on pixiv all use SAI).