PompeyP has it about right I would say air law then nav definately you cant solo without air law and you cant qual xcountry without nav . The PPL confuser is the best bit of kit going every question on every exam I took was in the confuser in one guise or another. Airquiz was useful but it depends what you want . A lot of the stuff in airquiz isnt in the exam but it does test your thorough knowledge because your not just learning the answers that you need , just depends what you wanna do .Read the books and use the confuser it worked for me , I am a bumbling incompetent but I managed to pass the exams ,
Good luck and enjoy