you will never forget your first solo, but for sheer satisfaction, the QXC is right up there too
For me the QXC was a much bigger deal than 1st solo. 1st solo was over and done with in a flash - the qxc was much longer and more challenging and is still the longest solo flight (added together) I have done - although I'm hoping to change that stat in the not too distant future. I got my licence in June btw.