The goal of an efficient thumbnail design is akin to a bait. It is meant to catch the eye... but not just any eye: the lizard eye. As you probably know, there is -- metaphorically speaking -- an animal that sits in waiting inside every YouTube user out there. Known to psychologists as the limbic cortex, the lizard brain is the part of the human intellect that reacts on impulse: quickly, and with limited input. The lizard brain is what allows the animal in us to catch the fly as soon as it appears within the tongue's reach.
Words are first in line when it comes to their ability to grab attention and ensure that the reader reacts. That's because lizards and other amphibians cannot read; but since most humans can, we can safely assume that the people browsing a YouTube feed can indeed read.
So what do we know about words? We know that words have both a meaning and a form. Words are formed out of letters, and these letters are shaped into characters by the font that forged them. From a designer's perspective, the meaning of a word can be altered, tweaked, and even skewed by the shape and other attributes of the letters used to put it together. Let's look at a simple example: where an otherwise innocuous word is highlighted in red in the middle of a sentence, it will bring attention -- and importance -- to itself, therefore changing the overall meaning of the context in which it appears. The same effect (or affect, as it turns out) can be achieved using fonts: a word appearing in a relatively large, bold font will automatically pull the reader's eye to itself at the detriment of those words that surround it.
Fonts affect the readers far beyond shifting the onlooker's attention. In fact, many of those effects are subjective. It is impossible to anticipate precisely the effect of a particular font on an individual since that effect is determined by personal, contextual, or even cultural factors; yet some effects are universal enough that they can be assumed, regardless of the particulars of the audience. You can read more about the psychological effects of fonts and how the shape of a word can be used to affect its contextual meaning here.
In short, because of the human factors involved in processing design elements such as fonts, there's no absolute rule for choosing the right font. In fact, there might not be such a thing as the perfect font. The method I use is simple and gut-driven, so to speak. I first select the affect I want the thumbnail to convey from the following list:
Looking at a selection of available fonts, I choose the ones that match those characteristics and get rid of all the others. This informal process usually leaves me with 3-4 fonts to choose from. Testing the remaining fonts in context, I quickly end up deciding on the best candidate by a process of natural elimination. I suggest you try out this process yourself and see what you can come up with.
Here’s a selection of fonts suitable for a typical YouTube thumbnail design, directly from the Google Fonts collection. Google fonts can be used on most web-based design platforms such as Canva, DesignBold, Befunky, and others. You can also download them to your local computer system for use in desktop applications such as Gimp, Photoshop or Inkscape.
Here are various sources of font suggestions for use in YouTube thumbnails: