Best opening for black to use

a2

Personally, I'm a fan of the French Defense. 

It's easy to learn, effective, and you can employ the Horwitz Defense for a Queen's pawn game (so it does double duty).  Although it is infrequently played, the Horwitz hasn't been refuted (yet)

The Caro- Kann is a good one, not as much theory as a Sicilian but rich enough in ideas to be a top chice of GM's since you don't create many weaknesses. For those reasons I play it occasionally.

Another good reason is if you need a defense to 1.d4 The Slav is the same in a lot of ways

Play d5 against d4. If your opponent plays a queen's gambit, play QGD semi tarrasch defense. If your opponent plays in any other fashion, but play QGD moves, i.e. Nf6 and e6, you get a solid, but winnable game. if you want ill post a game of mine in this wonderful opening.

oh, and against e4, I strongly suggest a hypermodern defense, so I would say either the modern, alekhine, or Nimzowitsch. The alekhine is the most sound, but you will have much less center, the Nimzowitsch is next, but there is little theory, just your skill and ideas to back it up, and the modern is still sound, has a good bit of theory, and can be aggressive. I would suggest the Nimzowitsch in the end. Hope I helped.

i used it in tournament once against a intermediate player but still lost, but the position arise from the opening isn't too complex like the sicilian as long as you know the first moves and didn't lose your knight, petroff defence is a sound opening and maybe you should consider it as your repetoire since the position is pretty simple to handle, i recommend the caro-kann too, same reason since it's easier to learn than sicilian, for the hypermodern opening maybe you should try alekhine defense for a surprise weapon, it's pretty good opening i have used frequently in blitz here, if the opponent didn't know how to handle it you can get out from the opening safely, but dunno about OTB tournament maybe the enemy already prepped against it

as for d4 maybe a QGD or slav might work, whatever suits you the most. if you like 1.d4 1.nf3 you can consider benoni defense if you want to play for a win since it offer a pretty sharp position, but the chance of losing is higher

The QGD has a lot of theory and the slav is passive. It depends on the way you like playing if you are to play the QGD, or else the variation which suits you may be horrible.

 The Slav isn't passive at all! If white doesn't play a lot of book moves, black equalizes by only playing natural moves, because he can develop smoothly. So I think passive isn't necessary the right word.

key words if white doesn't play book moves

white won't play book moves below 2000 elo

That's assumption.  I know the book  moves against Sicilian Dragon as a kid who doesn't play sicilian

Playing book moves isn't too uncommon.

I recommend the Caro-Kann or the French Defense against 1 e4 and the Slav Defense, which has the same basic setup as the Caro-Kann, against 1 d4. In all cases, the middlegame for Black often involves a Queen-side counter attack with beginning with ...c5.  The advantage of the French is that you get ...c5 in one move.  The advantage with the Caro-Kann and Slav is that your QB is not trapped behind the Pawn you'll eventually put on e6 and your setup is a very solid defense which is hard to crack, especially if White plays the Advance Variation against the Caro-Kann (1 e4 c6 2 d4 d5 3 e5).  The French is also strong against the Advance Variation (1 e4 e6 2 d4 d5 3 e5) when an immediate 3...c5 works.

What do you mean by too complicated?

Around 2010, IM John Watson wrote, "... For players with very limited experience, ... the Sicilian Defence ... normally leaves you with little room to manoeuvre and is best left until your positional skills develop. ... I'm still not excited about my students playing the Sicilian Defence at [the stage where they have a moderate level of experience and some opening competence], because it almost always means playing with less space and development, and in some cases with exotic and not particularly instructive pawn-structures. ... if you're taking the Sicilian up at [say, 1700 Elo and above], you should put in a lot of serious study time, as well as commit to playing it for a few years. ..."

Well, then I'll play closed sicilian whenever I face it.

Some of the suggestions above are great. I wouldn't Alekhine's defence, unless you really know it though. You want openings without a lot of easy tactics and attacks, closed openings generally are favored. But don't assume there wont be lively attacks, because that would be a mistake. Really, you need to understand the openings better and improve your ability to spot and avoid hanging pieces for tactics to hit you with.