I love most things, but I love underground rivers, don't like the sewers though, but the different types of build in underground culverts are awesome. Anything from brickwork to stone and concrete thrown in too. That's my favourite at the moment.
Also, love anything to do with either Military, or Railways, preferably derelict rail lines with parts of the track still in place, and other bits of railway stuff thrown in for good measure.
Like different industrial buildings, some huge like foundries etc, and some tiny one room industrial buildings, so many different types and sizes that I find fascinating.
Rural stuff, you find lots of interesting things that are still left in situ. Like finding an old farm or house or something off the beaten track, then when you wander in, finding old aga's, gorgeous fire surrounds and sometimes paperwork or books covered in cobwebs and dust.
And holiday camps, (that's cos i worked on a couple so enjoy looking around those). Spent 20 years walking all around the old Butlin's camp at Filey, watching it getting more and more derelict as the years went on, always wish I'd had the foresight to take a camera with me to take pics of it slowly deteriorating.
Excellent thread.
Sal