There are a plethora of different angles to aim at depending on what you're trying to hit and how high you are off the ground. For a basic ground level idea on what I aim at, view the pic below:
It's my basic low lung shot. If I were aiming at the heart, I'd be slightly more towards the front and a hair lower. I like shown aiming reference, because after the hit it leaves a good blood trail. When you have an entrance wound lower in the chest, it fills with blood and pours out easier. If you get a hit above the 1/2 way mark, then your blood trails are not as obvious, with sometimes being non-existent.
Picture the chest being split into 3 parts. I aim a hair under the lower third mark... or the top of the bottom region.(bottom 1/3)
iSnipe