When You Don't Know What To Play
... Play the Queen's Gambit Declined!
At least, I think that's what the Russians used to say lol.
Another example of how harmonious the black position is in Lasker's variation of the QGD. Everybody should have this as a 'solid' weapon.
Edit: It turns out my opponent was a Grandmaster, but with a FIDE rating of only 2392.
"The ability to work hard for days on end without losing focus is a talent. The ability to keep absorbing new information after many hours of study is a talent." - Garry Kasparov