For me, it's the knowledge. I was still in college when I first heard about Bitcoin. I learned some needed information after a few weeks of doing research and gathering information from someone I know. I learned about investing, its importance, and such as, about the technology, what and why it is created, and how it will be beneficial to everyone.
The knowledge I learned that time is still useful till this day. But learning about Bitcoin will not end there, in our daily lifetime with cryptocurrency, everyone should continue to learn and gather information as this technology also continues to grow time after time.
This is a nice answer, material possessions can come and go but knowledge will stay with you forever and you will be able to apply it in all kind of different circumstances, so if at some point you decide to diversify your portfolio and invest in an asset other than bitcoin, the knowledge that you have already will come in handy and it will allow you to become successful way sooner than someone that is just starting out investing in that asset and that has no prior experience.