We've used 2x12s. My husband adds a 2x2 at the bottom. The premise being that will rot first, and he can replace the 2x2 and continue to use the 2x12. Can't tell you if this will work we've only got 3 this way. If you live where there are voles, line the bottom with hardware cloth.

We used cement blocks. They work fine for us. Our reasons for choosing them were $$, durability, and ease of construction. If we'd had any extra lumber around, we would have used it; we liked the look of galvanized metal with wood frames, but again, the cement blocks were cheaper and easier. We stacked the blocks two layers high (about 16" high). Check all the options out online--there are all kinds!

I am thinking about adding a few new raised beds and I think I will try galvanized stock tanks. I am looking at 2 rounds 4'diam x 2'high and 2 round ends (oval) that will be 2'x2'x5'. You can paint them if you don't like the galvanized look. I have a large round that I put all kinds of plants in but it is in part shade the new round ones will be in full sun. I am hoping these new ones will help with my back.

Thanks