Court
Curved on the outside and slightly less curved on the inside to make it a little more comfortable.
Simple, classic and easy to wear and suitable for pretty much any ring size.

Rectangular
Another easy-to-wear classic.
I would tend to make them a little shallower as the ring size increases in order to keep it comfortable to wear.

Shallow Reversed D
Somethings called a 'comfort fit', this one is curved on the inside and flat on the top. For very small sizes you might want to go for the 'true D section' in order that the ring not be too flimsy.

True reversed D
This one has a section that is half a circle so, if the ring is 5mm wide, it will be 2.5mm deep.
Especially for larger sizes and widths, this can get a bit uncomfortable (and expensive!) so I'd suggest you don't go above 4mm for a ladies ring, or 6mm for a gent's. Unless you really like chuncky jewellery that is.

Deep Reversed D
I use this one in a lot of my silver and gold designs such as the bead torque and the plain diamond set bands. I wouldn't suggest going above 3.5mm width for the reasons mentioned above, but it's fine in a 3 or 3.5mm width.

Oval
These are nice, positive chuncky rings and I'll make them up to 8mm wide which constitutes a serious bit of metal. 4mm is a good size for a lady and 6mm for a man. These are the basis for the popular Gold Stripe rings and I've made these as wedding rings in various metal combinations.

Shallow oval
These sit a little lower on the finger and so are a little more comfortable to wear. The section is the shape formed by two intersecting circles and is a nice alternative to the traditional court section.