I've been to Lower Bay, Lower Queen, and Hillcrest, and by far the most interesting of the lot is Hillcrest. I don't know what's on the tour, but I'm hoping/assuming it will include the old transit control centre. It looks like something right out of a 50's sci-fi movie: lots of plate glass and panels full of switches, the highlight being a large wall-sized map of the transit system covered in hundreds of little lights to represent various mechanical systems.
Lower Bay is also interesting, but the appeal of that one is honestly more just to be able to say you've been there. Before the TTC lets the public go down there they're going to give it a big clean-up, get rid of the graffiti and debris, make sure all the lights are working, and in so doing they're going to basically ruin the post-apocalyptic appeal of the station. To really appreciate it you need to see it in its natural state, covered in a film of brake dust, with broken and flickering lights overhead and nothing around you but the echoes coming down the tunnels.
Both are worth seeing, but definitely Hillcrest is the one transit enthusiasts should make it out to see.
