I mainly watch vlogs / gaming videos, and I've found that I mainly click on:
Vlogs - Usually point out the best part to entice you to click on the video, for example - Roman Atwood's latest vlog is titled "Destroyed the christmas tree", with the thumbnail being them stood over the christmas tree with it laid on the ground.
Gaming videos - I've seen a lot of people using the "rainbow" thumbnail trend, but I find that big bold text is probably the best thing to go for.
Just don't use misleading thumbnails that have nothing to do with the video what-so-ever!!