As mentioned it depends on what you're looking for. There's four "main" types of sonar, standard 2D sonar, down imaging, side imaging, and forward facing. In addition to that, most graphs nowadays have some sort of GPS capability, allowing you to plot a track, save waypoints, and view them on a map.
2D sonar is what we have had for ages, fish will give a good solid return, but it is more difficult to identify various types of structure, it usually just shows up as blobs.
Down imaging is similar to 2D, but is much better at showing bottom composition and types of structure, and not as good at showing fish. It only works when the boat is moving.
Side imaging shoots to the sides of the boat, how far depends on the depth of the water, rule of thumb is 3x the depth. So in 15ft of water you will likely be scanning 45ft to either side. It will give you a very good view of the lake bottom and any structure that may be there. It will also show fish pretty well when it's set up right. Many people will run side/down imaging in split screen and scan an area at idle speed, doing this you're covering a 100+ foot swath of the lake, eliminating a lot of dead water. Similarly to down imaging, it also only works when the boat is moving.
Forward facing sonar gives you a 135* live view forward, down, and slightly to the rear of the transducer. It's still pretty expensive but nothing else really compares. It's almost like having a camera.
Best bang for your buck right now is the Garmin 93SV for $699. It has everything mentioned and the capability of running forward facing sonar should you decide to add it later.