It's hard for me to definitively state what my all-time favorite show is. It differs depending on what I'm watching at any particular moment; when was the last time I did a rewatch; how active I am in a particular fandom; etc. So I have instead simply chosen my all-time favorite episode of television ever, which happens to be from one of my favorite shows.
The X-Files - 5x05 - "The Post-Modern Prometheus"
Day 4 - An angry gif