There are no filler episodes
It’s a subjective term. Filler isnt a negative term. Fillers can be good or bad and they exist in every episode. Getting from point A to point B is every arc, wether it be season, series, or episode. In between which is filler. Things to keep you entertained while going from point A to B.
Its also a matter of perspective. In Series 2 the Seasonal arc was the destruction of the Universe at the hands of Mantrid. A few episodes made no reference to this fact.
Some episodes like “Wake the Dead” had really nothing to do with finding a home, mantrid, or history of the chars. But it was a DAMN good episode. One of my favorites and of course it introduced us to Pat Z.
The Vlad arc could even be considered filler but it was dang good as well. I think its just a subjective term and if you perceive it in a negative way then you can argue “There are no Filler episodes” if you think filler is just things total out of context.