I think that would depend on your playing style to be honest, there are a lot of good openings to choose from.
What opens do you think works best and why?

I believe for lower/middle rated players its important to play solid strong positions and stay away from sharp lines and gambit lines such as Kings gambit, Scotch Gambit, Benoni Defence, Najdorf and stick to lines that will get you safely into the middle game like the Catalin, Ruy lopez, French (although the French can be a little cramping), Kings Indian attack or defense and the 4 knights opening. The reason behind this is with sharp lines you have to break some principals in order to get/keep an advantage. These principals are more important than any opening line and they are;
- Control the Center
- Develop
- Keep your King safe.
Those three rules are more important to know the ANY opening line. Hope this helps.
I'm looking for ya'lls opinion on chess opening's so I would like to know what you thinks.