What do you mean by “key issues”?

Just important comics

What do you mean by important comics? Do you mean comics that changed the DC Universe as we know it? Comics that introduced major DC characters? Comics that made a lot of money in the first week of sale?

There isn’t a list of generalized “essential comics” for DC as a whole after all. Many mythos within DC were independent from each other in many ways. Crossover between the different sections of teams were usually limited to team books and team-up titles.

Let's ask @TFF2! . It's more of opinions on what stories fans think are important

So Flash #123 is the Flash of Two Worlds, I think The Trial of the Flash and the Wedding of the Flash comes to mind

Though I have not read any of those stories

One of the important universal concepts introduced before the Crisis is Krona and the origin of the universe, as explained in Green Lantern Vol 2 40. I don't know if that fits your definition of important.

^Ooh! I thought it was exclusively for Flash 😅

Let's see now. A story that's quite famous if I do say so myself is Superman's Return to Krypton, where Superman meets his birth parents and falls in love with a Kryptonian actress

Another important issue is Superman and Batman meeting for the very first time

I myself just got into the Silver Age not too long ago, so I'm still not so knowledgeable about pre-crisis stories

I was adding all of the golden, silver, bronze age comics and calling them pre crisis sorry for the confusion