Brightboys, there is no "best" construction: the reason there is so much different advice out there is because gardeners have such different requirements and our gardens have such different spaces. And, of course, we have different sized budgets! As you can see, a large part of my garden has been concreted over by a previous occupant. I wanted more growing space, and I thought building raised beds on the concrete would be easier than breaking it up. There are two more beds like the one in the picture, out of frame to the right.
The builders' merchant was very helpful (Richard Williams of Llandudno Junction), cut the wood to the size I wanted and delivered it onto my drive. As well as the blocks you can see, there are vertical square-section wooden pegs inside the corners. The construction was dead easy, all I had to do was drill holes in the pegs, and drive weatherproof screws through the holes into the blocks. All my own work, and if a little old lady like me can do it, you can. Building the three took about 6 - 8 hours as I recall, and cost just under £200.
I broke up the concrete in one of them, then had second thoughts, because my non-gardening neighbour has uncontrolled bindweed and it can travel underground. I thought if the bindweed comes up through the raised bed, I'll never get rid of it, so I left the concrete intact in the other two. It doesn't seem to make any difference, the veg grows just the same in all three.
Last year I was advised to cover, in the autumn, my beds with flattened cardboard boxes which supposedly rot down in the winter and the worms incorporate them in the soil. Despite living in a high rainfall area after six months the cardboard was still intact and a ideal harbourage for slugs and other pests.