When I want to play with cake, I make it myself, and bake it just until it sets well enough to hold all the frosting in the world. That way, it makes a much gooeyer mess.
I prefer chocolate with wild colors, as it coats much better than white cakes.
I will also soak the cake in chocolate syrup while it's still warm to add to the gooeyness.
Costco cakes. Practically made for wam. Deep, gooey, soft sponge with plenty of soft frosting, and large. Can't get enough of smashing and sitting on these beauties.
Yes, for me it is important that the cake looks pristine and perfectly decorated before it gets squished. Don't know why - I guess it's the naughtiness of ruining something that's perfect.
For me, it depends entirely on how the cake is being used. If it's getting smashed into my face then I actually like drier cakes with lots of frosting on, as the frosting sticks to your face a lot better and it feels great!
If I'm sitting on the cake (which is usually where cake ends up with me) or being rubbed anywhere else, then the gooier the better. Something like Gateau or a cake with a lot of icing. When you flatten a cake, it's surprising how little mess it creates so the gooier the better, certainly for coverage.
I'll usually buy them from a local shop or bakery (Cheap enough most of the time), however for special occasions I'll make a cake and decorate it.
As for the bonus question: Yes, it does matter. I love a cake to look all pretty and pristine before it gets squashed, especially when it's homemade. I think it's knowing that someone has spent so much time on decorating it and making it look perfect, and it's about to be sat on and completely destroyed.