Your best investment will always be YOU. Regardless of who signs your paycheck you are self-employed, selling your services to the highest bidder. Here is the formula that I have used for 25 years to constantly improve the value of my services.
#1 Read one per day in your chosen field. Reading one hour per day equates to about a book per week. This will build your reservoir of knowledge in an exponential way.
#2 Invest 3% of your gross income each year to improve your marketable skills. Attend seminars, invest in classes, buy books and audio programs.
#3 Make all driving time, learning time. Listen to self-improvement programs while you drive. Learn a language, learn a skill, or get motivated.
Improve upon yourself a little bit each day and your skills and value will compound.