I have one for "worst". The illustrations for the Wortenia War series are so bad compared to other published series.

And for best I think so-bin's work on Overlord is outstanding from the rest.

The Overlord illustrations can also be my pick for "weirdest":

For the funniest illustration, I clearly remember an illustration of Rozemyne driving -proud as can be- in her highbeast through the castle halls with either Ferdinand or one of her retainers walking beside it with an exasperated expression. I am just not sure if this is really an illustration or if I only imagined how a scene from the books would look illustrated.