Well well, in my country the dollar currency is not the official currency but in an attempt to answer your question, I'd covert the $20 to my country's official currency worth and as at today that would be somewhere around 7000 naira.
The best $20(7000 naira) I've ever spent was spent on getting a recording device for my journalism work.
It's a video camera eye glasses that allows me to perform the work of an investigative reporter undetected. And as you know anonymity to a large extent is key in investigative reporting.
Although, I'm still a student in this aspect of journalism but having this gadget is giving me an edge over my peers, I could get the stories from people who would not be willing to talk if they have the idea that they are being interviewed but what I do is just strike a normal conversation tilted towards the subject of focus while my recording glasses captures the video and the audio without the awareness of the person because it's a normal eye glass on the outside.
In a nutshell, since I spent this money to advance my career and professionalism in my chosen field of journalism I think that's the best $20 I have spent because it's an investment into my career and future with a massive ROI when the time comes.
The best 20$, or the equivalent of $20, that I ever spent on something was a trimmer. I was somehow forced to do that because I moved to a more expensive city and I had as usually to trim my hair about two to three times a month and it would cost me some good amount of money. Having a trimmer at hand and the guts to trim my hair by myself it was perfect. I have even now after about six years that trimmer and still do my hair by myself. This is really my best $20 spent that helped my save a lot of money and time by not gong to hair salons anymore.
The best thing I've ever done with $ 20 is to give it to "street singers" who look very hungry. I use the money to buy food, drinks, and give the rest of the money to go home.
Charity of good deeds will always be pleasant.