Backplate and wing
Most experienced divers have switched to a backplate and wing and there is little reason for everyone not to follow them.
The Backplate
Backplates are made in a variety of sizes, but the standard size will work fine for most divers. You don’t have to worry about gaining or losing weight and you can loan your bcd to just about anyone. It’s very convenient.
They are commonly made out of four different materials: carbon fiber reinforced plastic, aluminum, titanium and stainless steel. My general recommendation is to stick with stainless steel. They have lightweight ones under 2.5 lbs for traveling with and diving in warm freshwater up to 10 lb ones for giving you extra weight for diving in the cold ocean.
The Wing
Wings are a little more tricky. If you are diving in the ocean with big waves you might want one with more lift to keep your head further out of the water. If you are traveling and diving in warm water you might desire the smallest one you can find.
The Harness
Most of the parts that companies want to charge you hundreds of dollars for on a harness just get in the way. Simpler is usually better. I have a whole box of different harnesses I’ve replaced with a simple continuous piece of webbing.
The one thing to worry about is rubbing. The harness rubbing on your exposure suit can quickly damage it. I use a neoprene sleeve on my crotch strap for this reason. At one point I used a chest strap and it just caused more wear on my wetsuit.
The most basic harness has two bent d-rings around the clavicle held in place by belt slides and another d-ring on your side to attach a SPG.
Straps:
D-rings
Bent D-rings
Belt Slide
Serrated Belt Slide
Cheap Aluminum Backplate
Titanium Backplate
Lightened Stainless Steel Backplate
Basic Stainless Steel Backplate
Heavy Stainless Steel Backplate