If you know nothing about your opponent, you should pick the opening and playing style that has achieved the most wins for you in the past, regardless of how many draws and losses you might also have incurred playing that opening.
What you trust and what you use
Sort:
Whatever you're most confortable playing with. It's pretty pointless to have an alternate opening for different situations if your not proficient in either. Learn one well and you'll win with it.
I ment what YOU use... so that you can share it with us. It was not meant to be of help to me.
must win? probably the Gruenfeld against d4, i dont really have a must win opening against e4.
Play whatever you think your opponent hates the most.
This works well on your local club because you know your opponents , their style and what they like to play , but in a tournament you somehow have to figure this out .
I think that knowing ay least 2 book-openings and to 2 book-defences will be enough to give you the weapons you need for any fight.
So depending on your mood,tournament-standing or preparation, you deside what opening/defence weapon to use.
I happened to pick these basic weapons :
As Black on 1.e4 -> 1. .. d6 or 1. .. g6
As Black on 1.d4 -> 1. .. Nf6 or 1. .. d6
With the white pieces I play both 1.d4 and 1.e4.
When realy having to win : Morra-gambit,Benko-gambit,KingsIndian or other , lesser known QueensPawn stuff.
For black Sicilian Najdorf (1.e4 c5 2.Nf3 d6 3.d4 cd 4.Nd4 Nf6 5.Nc3 a6) or a Dragon (... 5. ... g6) is a good choice for 1.e4 and for 1.d4 King's Indian or Gruenfeld.
For white I try to avoid exchanging lines leading to endgames, playing 'normal' solid game and hoping for the best.
I agree with pelly13 that it also depends a lot on your opponent. What kind of play is he strong at? Does he calculate well or prefers more positional game? Play what your opponents feels uncomfortable with is a good idea.
While I have played everything under the sun, from the Queen's Gambit to the Orangutan, from the Caro-Kann to the Latvian Gambit, I have a select few I play in major tournaments:
White: 1.Nf3, 1.d4
Black vs e4: Caro-Kann, Modern, 1...e5, or rare occasion the Czech Defense/Pribyl
Black vs d4: Basically anything 1...d5 other than QGA, e.g. Triangle, Slav, Semi-Slav, Tartakower, Tarrasch, etc., and occasionally the Modern Defense or No-Name Defense (1...d6)
In smaller one-day tournaments, or at the club, or against players I play all the time, I might occasionally play 1.c4 as White, or the Scandinavian, French, Sicilian, Sniper, or King's Indian as Black.
Of course, in the course of our chess playing time we use and experiment with various openings and playing styles, sometimes going all tactical or sometimes curling up in a shell. However, when you are in a must win situation or in a tournament, which opening or playing style do you prefer to adopt? Please post it in this forum.