My favorite thumbnails are the simple, classy, easily recognizable ones that also clearly indicate the topic to be covered. Basically, something that will grab my attention and tell my brain “Hey! A new video from that channel I love! I will enjoy watching this and this is what I will be learning about today!” At some point the reaction really becomes almost subconscious–see 8-bit music theory, DF Retro, LGR/LGR thrifts. A good channel plus a distinctive font or color choice really just directly stimulates my lizard brain (can’t speak for the algorithm’s brain, though). The silly face trend is disheartening, but in your case I’ll allow it.