In the reverse of what is normal for lounge suits, single breasted dinner jackets are generally considered more formal. Although notched lapels are becoming more common, it is smarter and more interesting to take a step away from the lounge suit and towards the tail-coat by having peaked lapels. A slightly less formal alternative is to have a shawl-collar. This harks back to smoking jackets and it's smooth line and almost exclusive use for evening wear makes it a very elegant choice. Either way, the lapels should be faced in silk: either satin or grosgrain, although grosgrain is often viewed as more sophisticated. Occasionally you see two-buttoned jackets, but the deep V shape of a one-button jacket is far more stylish and traditional.
This sort of jacket is classic and very smart indeed. It does, though, require some sort of waist covering, either a traditional waistcoat or a cummerbund - more on which in another post. Hackett and Ralph Lauren offer both peaked and shawl collars in classic styles and beautiful cloths. For a slightly less pricey option, TM Lewin are well worth a look (although they don't seem to show it on their website).
The less formal alternative is a double-breasted jacket. These can come in a number of button configurations but a single-button fastening is common, either a 4x1 as above or even the very bold 6x1. Double breasted jackets are, in many ways, a more comfortable option especially as they don't require a waistcoat or cummerbund. The rare option of a shawl-collared, double breasted dinner jacket is as informal as it gets, merely one step away from the smoking jacket. Nevertheless, these days it could be worn to almost any black tie event and people would be more likely to admire your style than think you inappropriately dressed.
A double-breasted, shawl-collared, 6x1 jacket. As close as you'll get to wearing a dressing gown to dinner.
Double-breasted dinner jackets can be harder to find off-the-peg, but it's not impossible. Brooks Brothers offers one at a very reasonable price.