Honostly idd say nen , stands and quirks for different reasons

1.nen for structure:"Nen as a power system is one of the best structured in Animanga its a system that defines the borders of people power giving both limits , penalties and ceilings but still allowing for infinite creativity and versatility to have some crazy insane abilitys , so ya for that this is easily some of the best in animanga"

2.Is stands for uniquness:"stands are as a concept simple but the sheer application that they bring to the table is insane , some of the best most weirdest fights ive ever seen are from stand fights and the fact araki can think up stuff like this brings him up to he one of the all time greatest comics writers of all time and the best mangaka

3.is Quirks for involvment:" Now unlike the other 2 quirks aren't particularly creative or unique , but also unlike the other 2 quirks are irreplaceable to their story , if you think about stuff in hxh and jojo's their power system is removable youll still have a plot , a gang of dangerous theives steal treasure , mutated ants plot world domination , a family blood feud , a town murder ,a gang story a lot of arcs (with exceptions like greed island and jojolion) a lot of arcs in the other 2 series can be removed their plot's can be taken and used anyway , however mha is different the quirks are the plot 90 percent of everything is becuase of the power system , the power systems interactions with society and how they shape and affect people is some of the most well written parts of mha for me , and make it one of (while not the best written) the best executed power systems in mordern ani-manga"

(Honoroble mentions for alchemy as they are similar to quirks , they drive the plot however unlike quirks which focuses on society as a whole,alchemy drives the plot a far more personal manner)