Where is the best place to have a raised bed next to metal building?

You don't say where you're located but I would go by your area and how hot it gets there. I would then check the plants you are interested in. Some like morning light and some like stronger/hotter light. I plant plants that like it cooler in the east size and the ones that need more heat in the west for afternoon light.

Generally, a garden with a south-eastern exposure will get the most sunlight during the day, but that will depend on where you are and what the tree cover nearby is like. It'll also depend on what you want to grow. Some things like a lot of direct sunlight, while others might need a bit of shade. Do a little research into what you want to grow and how much sunlight it needs, and then choose your direction based on that.

I grow several different kinds of plants in my garden every year. I prefer to use morning light for my plants. So I would choose to face east. When you're ready to purchase your plants, take a look at the little ID card that comes with the plant. It will tell you how much sunlight that particular plant prefers.