If you are looking to get engaged and you aren't sure what type of ring your fiancé would want, it's important that you get an idea before you go shopping. Purchasing an engagement ring is a big investment and you want it to be something that she wears forever, not something that she doesn't like and doesn't want to show off. There are a few ways you can try to reduce the chances of getting a ring that isn't liked, and to get something that she will like.
The diamond is a classic engagement stone. Get a ring that has diamonds or a single diamond, instead of picking something with a gemstone or other types of stones added into it. Something that is classic will look timeless as long as she wears in, instead of something that is a color she is into at the time, or something that will go out of style later on. If she later decides she wants a different band or style, she still has the original diamonds for the new design.
White Gold or Sterling-Silver Band
Some people like the look of things like platinum or traditional gold, but white gold or sterling-silver are neutral and liked by many. Avoid anything that is engraved or has an odd shape, since this is something that may only be stylish at the current time, and that may not be favored later on. The band size will be adjusted later on.
Avoid a Set
If you get an engagement ring that doesn't come as part of a matching set, then another band can be purchased at a later time. The other band can be custom picked out by the person wearing it, and they may pick a band that is more like what they originally imagined for their engagement ring. They could even do two bands, one on each side.
You may not be sure what to do to pick the perfect ring, but as you're looking you should start to see rings that look like they match their personality, and that you think they would like. There are a lot of different rings to choose from, and you can choose some that have both round and square diamonds, or that have elevation but aren't just a single mounted stone. Shop around until you find the ring that speaks to you the most, and that you are instantly attracted to.