Definitely object oriented design, in a negative way. It taught me how I do not want to think about a computer system. All its ideas sound so neat and if you have a well designed system, which never needs to change, it might even work. But someday, somewhere you might realize that some functionality means none of your abstractions made any sense. The idea of combining functionality with state (classes) was definitely a useful one, but you shouldn't use a hammer as a screwdriver.

I have a lot of positive things to say about procedural programming. It is perhaps the easiest and most straightforward way to think about computation. The more abstract the problem gets the more I start to think about it in functional terms, it allows you to construct the flow of data in a very managable and efficient way.