A corner mark or a tiled one
Pick by what you are actually trying to do. A corner mark signs the work and stays out of the way, and anyone determined can crop it off in one move. A tiled mark is spread diagonally across the whole frame, so removing it is far harder than cropping a corner. Use the corner for a portfolio, the tile for anything you are showing to a buyer before they have paid.
Settings that work
- Size around 5%. Big enough to read, small enough to ignore. Go to 8 or 10 if the mark is the point.
- Opacity around 55%. Fully opaque text looks like a mistake. Too faint and it disappears on a busy photo.
- Bottom right. A common choice, and rarely where the subject sits.
- Use a domain. "yoursite.com" is something a person can act on. A name on its own is not.
The watermark is applied last in the chain, after any crop or resize, so it is never chopped in half or shrunk into mush.