I've been doing this too long and have taken way too many photos so it's difficult to think of an outright favourite however I have a few that do immediately come to mind when someone asks me this question.
HF6 Blast Furnace, Liege (now demolished). I had wanted to get this shot for so long.
Dannah House Care Home, Wroughton near Swindon - I never would have predicted that I would come out of a soggy ruined care home with one of my personal favourite photos.
Abandoned mansion, USA. The sexiest staircase I've ever shot.