Building above ground garden, What is the best materials to use?

Cedar is the best wood if you are looking at wood, but they also make a ton of composite woods that require way less maintenance!

wood wouldn’t be easily made into a smooth circle, but perhaps a very interesting angular type Circle? Otherwise I would go with landscaping bricks which will make a perfect circle.

I have a raised garden and I love it. Mine is made with landscaping timbers. It's in the shape of a rectangle. Put down weed fabric before adding your soil. My garden is made with a mix of one-third soil, one-third manure and one-third sand. The sand is for water retention. This soil mix is great. I plant a lot of tomatoes. I start the first row with the tomatoes that will produce first. The second row will be the next maturing plants. This way I have tomatoes maturing all season long. I use a soaker hose to water. Really it's just a long hose with pin holes down the entire length. I wind the hose in and around each plant. I water the soil and not the plant. It;s called deep-root watering. And last, I want to talk about pollination. As you may know, we are in severe shortage of bees in this country. You can self pollinate by gently jiggling each stalk yourself. It only takes a couple seconds to do this. Check this out: unpollinated plants will bear 10 to 15% of its capacity. Pollinated plants will bear 90 to 95% of its capacity. Crazy, right? Sorry this is a bit long. I love to garden. Best wishes.

I would go for either a Brick retainer wall or one made from Railway Sleepers either laid in a brick fashion or sunk in ground upright and back filled. Any gaps between Sleepers could be filled with a plant to help naturalize it...........

Only a "save the knees and back suggestion", I built my raised garden so that there was 3' of bed, a space wide enough to turn my foot, then the side of the next bed. Thus, a board placed across the sides of the two different beds, you can SIT and garden, and 'scoot' from one end to the other. Soon 87 and have done this for 31 years. I had fir sides to the beds, many are about rotted out, but lasted that long.