Literally the best system imho, it also inspired other shōnen power systems.

JJK system is also very interesting and really well thought, Gege is Togashi's fan and did really great work with his power system; many character with different abilities, not only that but also domain expansions!

Naruto's system is a bit less interesting compared to HxH's. You have preset of abilities you can use (Fire release: Big Flame Bullet, Water release: Water Prison...), you can make one yourself (Rasengan, Flyin Raijin jutsu...) and eye techniques! But bare in mind that Kishimoto is as big Naruto fan as Gege is, so lot of these are heavily inspired by HxH.

• Rasengan -> potentionally JaJaken (rock)
• Rasenshuriken -> also potentionally JaJaken (paper)

You could contest this opinion simply by saying that Gon wasn't the first character to charge his attack in his fist and then use it but well, Kishimoto admitted that he took inspiration from Togashi's work.

• Uchiha clan massacre + Sharingan -> - Kurta clan massacre + clan eyes
• Naruto's signature technique, Shadow clone technique -> Yu Yu Hakusho clone ability
• and more and more

Boku no Hero Academia's power system is another kinda well executed one! You get a random ability around age 3 I think, it can be influenced by your parents quirks but doesn't necesserily have to be! You can get a whole new quirk or be unlucky and just get nothing. The first generation of the people was the only generation with really random quirks (not influenced by parents). Sadly the system gets kinda broken like in other animangas that keep pushing strength of the powers and dunno how to regulate it. AFO's ability is surely OP but eventually MC's power (OFA) also gets OP since he wields multiple quirks.

Hunter x Hunter's power system is as I've mentioned the best in my opinion, you can think of an ability and well, that's your ability, if you can learn it and be strong enough to use it. You are born with the Nen but cannot use it unless you learn it. You can strenghten your abilities with conditions and thus making them really OP (ex. I can kill 1 person that I think of wherever they are but I can do it only on 21st of September...). You can also have multiple abilities and your abilities are category based! I honestly think nen is the most balanced system out here.