You will find a multitude of styles and materials when it comes to harnesses...but what's the right one for you or your lady?  Though it's not always possible, I always recommend trying one from each category to get a sense for what works best with your body type.  The basic one to start with is the Double Strap style harness...most couples will start there because of the price and versatility.  They adjust up and down to most body types and sizes.  Panty harnesses are great but tend to fit small so any ladies with a bit of hips or tummy might feel they aren't super flattering but the right fitting one will feel super comfy and secure.  G-String harnesses can be a little flimsy for those of you who like to really give it to your man but they are simple to put on if you can find one that works with your body type.  Below are some suggestions:
Double Strap