I'm a sucker for world building, and Gridlock is the best example of that in Who for me.
Tremendous character work sketched quickly and efficiently, great acting, the best closing shot in the history of the show, advancement of the series' arc in a meaningful and interesting way, old obscure monsters used in a creative new way, imagery, thematic depth, the slow-burning horror of what happened to the upper city and the length of the traffic jam... I just love it so, so much, in all its bonkers, creative beauty.