This thread hurts.
Recent past/present: Reed. Can you imagine our teams of 2001-2008 with Reed in place of Williams. Not to mention the other players we could've selected after trading down from 8 to 28 that year.
Past: My initial response was Montana. I consider him the great player of all time at the games highest impact position. But if you go back and examine to time frame you'd see he just wasn't in the cards for us.
We had an established veteran QB in White at the time who was just taking over for Staubach. And the player we selected over Montana, (Doug Cosbie) was also a great player for us. Would we have been better off selected Montana in 1979? Of course, but you can't cry to much with how things turned out.
Now Rice and Moss are a different story. I believe they are the 1st and 2nd best players to ever play WR. Both fit an obvious need at the time and I was certain we were going to draft both of them. That's some serious split milk there.