How can one be among the best of Muslims, enter the highest Paradise, and be the best and most beneficial to the Islamic nation, especially for those who study law, and what are the practical steps to achieve that?

Gaining Allah Almighty's love and closeness is achieved by observing the obligatory duties (fara'id) and refraining from forbidden acts (muharramat). Then, it is by striving in supererogatory acts of worship (nawafil) and avoiding disliked acts (makruhat). The more a servant strives in obedience, the higher their status becomes. This is realized by learning the Sacred Knowledge, then acting upon it, then inviting others to it, and then patiently enduring harm, which is what Surah (definition) Al-Asr encompasses.

As for the law: If it is derived from Sharia (definition), then the way to serve people with it is by acting upon it, applying it, and inviting others to it. If it is a man-made law, then you must clarify its contradiction to Sharia, explain the superiority of Islamic legislation over it, demonstrate the flaws of these laws, and call people to abandon them and return to judging by the Book of Allah and the Sunnah (definition) of His Prophet, peace and blessings be upon him.