You can construct a raised bed out of anything you like. I've seen them from corten steel, galvanized or corrugated metal, concrete blocks (CMU's), pressure treated wood, non-pressure treated wood, synthetic wood.....you name it. I've even seen raised beds made from kid's wading pools and galvanized stock tanks. The material will not affect plant growth.

The "best" is whatever works best for you.

Thanks!

Arguable. :-) Wood's great, but treated wood that isn't American/Canadian/EU could be mildly problematic for a vegetable garden (modern pressure-treated wood is fine; if using older, recycled, or imported from dubious countries, ask and check what was used). Unsealed wood will, of course, age almost visibly, but some people like the look. I don't object to it.

Unsealed woods built up will decay over time as well, so I don't recommend them for taller builds as they'll rot and eventually let go. It will take a while. But it'll happen eventually.

Metals decay as well, but sealed or painted metals are fine. Some metals are also problematic in contact with soils and roots (we occasionally argue about this here, but my philosophy is not to take chances or to overload soils with contact with metals we know to be toxic in excess). But if sealed or painted, again, no problem. I emphasize this because somebody's going to miss that. Whether you like the looks is, again, your decision.

Concrete is yet another option. It will alter the pH of the soil (I've seen claims that it merely leaches calcium into the soil without altering pH...but releasing calcium is a cause of changing pH, so...). Again, it can be sealed if you wish to avoid that, and UV-resistant sealants will preserve concrete color quite well but do need to be renewed occasionally as they aren't immortal. Or let it go and let the concrete pale naturally. Paint a mural on it for winter. I can recommend some wonderful lightfast mural acrylics.

Trex composite is the next on the list. Expensive, but it looks good, wears fairly well, and does a decent job. These are more "landscape edging," but if you only want a few inches of raised bed over grade, it's great, attractive, and durable. Handy people can build this much higher very, very easily, too.

Plastic is another, and probably my least favorite. Plastics tend (but not all types do tend) to fare poorly in sunlight over the years and decades without extraordinarily good UV coatings. Even so, those do break down over time, leading to the plastics chipping, discoloring, and becoming weak. Some plastics may leach into soils, but those plastic types are not generally used for this type of endeavor.

So it depends on your budget, aesthetic, and the height of the bed you're looking for. Economically speaking, wood is probably your best bet for greater heights when combined with durability.

What did I use? Nothing. My raised beds start at grade and rise, dome-like over grade as you move into them, up to about 2' at the patio, 1' at the house edge (where they drop back to grade to drain around the house), and 1' at the driveway (ditto on both sides). Unconstrained, the bed edges do walk a bit, though, but I like the open aesthetic.

Really, the choices are endless and for anything with any problem, there's a solution.

I was advised to treat raw wood with linseed oil. And thank you profusely for such a thoughtful answer. Cosmic hugs!

Linseed oil (used with oil paints, so I know the stuff well) works, but does wear away and is a food for bacteria, mildew, and bugs (claims that it kills or repels them have been mostly unfounded except for some wood worms). I'd recommend a commercial sealant, really. We've lost more art over the ages to rot and ants and beetles... Sigh.

You don't mention where you are, so keep in mind corrugated metal doesn't work in hot climates. It may be beneficial in the north, but in the Sun Belt it will get as hot as a car hood.

I’m in western NC. Good info, thanks.

Paint it white. :-)