Which program did/do you use for final design(s)?

Personally, I don't see it as choosing one program. Think of it in terms of programming languages -- which program is best for the task at hand. From Treehouse, I learned to use mock-ups programs like Sketch and Balsamiq to create a general idea of what I want the layout to look like. Sometimes this may be enough to get going, or for the entire project. Other times, I'll use Photoshop to create the layout with more detail, simply because I find Photoshop's interface a lot simpler to use. The only time I actually use Illustrator is when I need to create something like a logo or badge that will actually be used on the site. I use Illustrator for this because I prefer to have logos and such in vector form. However, you could save multiple versions of the logo or badge and render them depending on screen size.

It's ideal to just find a workflow that you're comfortable with. My way may not be the best approach for your projects. However, every handyman should have a nice toolkit. You never know when you need a flathead instead of a phillips screwdriver.