Here is the list of the head; series in order :
1 : CHAOS;HEAD NOAH (Remake of CHAOS HEAD original game so i will not put this one as first)
2 : CHAOS;HEAD Love Chu☆Chu! (Kinda a spinoff and not important if i understand well)
3 : CHAOS;CHILD (take place 6 years after CHAOS:HEAD NOAH and is a direct sequel)
4 : CHAOS;CHILD Love chu☆chu!! (Another chill spinoff wich take place after the True ending of Chaos:Child, it's a bonus pretty much)
So if you don't want to play filler play HEAD NOAH and CHAOS CHILD.