what is the most effective opening?

chunhee
what kind of opening did you usually do and master it? And why have you chosen that?
caryilchess

for white london system and for black if white plays e4 sicillian and if white plays d4 kings indian

woollensock
for white , h4 the woollensock gambit
for black , a5 the anti-woollensock defence
ChessBoy513

White: QP(1.d4)

Black: Sicilian or Caro-Kann against KP(1.e4), Semi-Slav or Queen's Indian against QP and 1...d5 or 1...e5 against flank openings.

MakeDeal

It depends on your playing style.1. e4 is most common for attack players while 1. d4 and 1. c4 are more commoj by positional players. With black agaionst 1.e4 most attack players prefer 1. - e5 or 1. -c5 while positional players more often play 1. -c6 or 1. - e6

GM_Kenny_Ji

g4 as white, and g5 as black

Ziryab

1...e6

willitrhyme

White: symmetrical, open positions.

As Black: anything which has 'Nimzo' in its name for 1. d4 or 1. b6/g6 for KP games

Pulpofeira

QGD Carlsbad as white and Petrov and Lasker defense as black. Master them, maybe in another life.

novacek

Arguably, at any level below 2000 the best opening is whichever one you know the best. So you should preferrably know a few lines at least 6 moves deep, be able to recall the plans (for both sides- so that you are able to both pursue your own ideas but also stop your opponents'), be aware of any tricks and traps you may come across, and have seen a few model games in the opening to reinforce your knowledge.

Oh yeah, it should probably be sound too. No point in playing an unsound opening and having to change it every other week.

staples13

The Sicilian Alapin as white 

IcyAvaleigh
as black I like the caro kann. it's simple and strong
jitterbugflapping
It depends on your play style
StrikeChess458

London as white,

wajeya

QG as white , caro against e4 and semi slav structrue against anything else.

Geodesist216

KIA for white , Pirc if e4/King's Indian if d4 for black.