Which shelf support works best? It depends. The first thing you need to decide is whether your shelf is going to have a back or not. In most cases, you probably want one, since it significantly strengthens the overall structure. But in some cases, either for decor or because you need proper ventilation for electronic equipment, you may want to leave it off.
Adjustable shelves are convenient, but they don’t add much strength or stability. You can drill holes and insert hardwood dowels or plastic or metal brackets to support the shelves. Or you can router a vertical groove, attach metal tracks, and insert clip supports. With both these methods, the key thing to watch is that the holes or tracks are positioned accurately, so the shelf sits level.
Fixed shelves are — to varying degrees — stronger. A basic butt joint screwed in from the upright will hold up the shelf and add a little strength, but you’ll have to countersink and plug the screw if you want it to look nice. Metal L-brackets or wooden cleats screwed into the upright and the bottom of the shelf are stronger. But these, like adjustable supports, are visible (and wooden cleats can get in the way of books).
The strongest and most seamless method is a dado joint. Just router a groove in the upright, and glue in the shelf. You can make the joint invisible two ways. Stop the dado 1/2 inch from the front of the upright, and install your shelves with a slight setback. Or add a face frame of finish material.
If your shelf doesn’t have a back, you should install at least one dadoed shelf to add strength and stability.