What is the best opening for a beginner who wants few pieces on the board? I am a beginner and someone who likes less pieces on board. I don't like complexity and have weak point for knights. My rating is 950 rn and has touched 1010 at best. 

So far I have used the Stone defence when playing with white and the King's Indian defence when playing with black.

Needless to say that defensive openings increase complexity. 

What is best opening for black and white for me ? 

I don't mind pieces exchanging. I just want less pieces on board that helps my mind process faster.
If your rating is around 1000, you should definitely abstain from the King's Indian for now. It is not a bad opening by any means, but it indeed is one of the more complex ones (requiring some sound strategic knowledge) .. too complex for your current skill level most likely, no offense. 

That said, there is no "best" opening for beginners or anyone else, everyone has their own taste and has to find out themselves. You have to try different ones and make up your mind what works best for you. In general, focus more on learning the core principles of opening play (development of pieces, fight for the centre, etc.) rather than on memorizing exact variations. As long as your opponents don't play along (because they have no clue about the "correct" continuations), they are not of much use anyways.

You should definitely have a look at the category of the open and semi-open games (everything where White starts with 1.e4) instead of the closed ones you tried so far. Each opening has its own sharp or quiet lines (and your opponent has a word to say on that too!), but 1.e4 openings generally have a tendency to be more on the sharp side and are usually advised to be learned first by beginners because they are more educative at this level (less strategic and more tactical on average). 

For the pinnacle of aggressive openings, there's the group of gambits. They do have some added complexity though because you have to be very accurate to maintain your lead in initiative (if your attack ever loses steam without getting the gambitted pawn back, you are in a bad spot) unlike the "mainstream" openings where a standard developing move usually doesn't lose on the spot (unless you fail to parry an urgent threat). Most of them are not "sound" enough to be played at GM level, but that does not have to concern you as you will rarely play against GMs. You have to do more preparation of variations than in other openings, so I suggest you don't start with gambits until after you tried the conventional openings for a bit.