Welcome to the site!
Here are some of my favorites:
Superman : The Sunday Classics : Strips 1-183, (1939-1943)
This is the golden age Superman at his most fun outside of the Cartoons. The artwork looks better than the comics of that time.
Superman: Man of Tomorrow Archives: Volume 1
Vol.1 of the Sliver Age Superman Archives! A must have!
Superman From the 30S to the 80S
This is an old classic book, mostly B&W reprints, you can get it cheap on Amazon.
Jimmy Olsen - Volume 1 : Adventures by Jack Kirby
Jimmy Olsen : Adventures by Jack Kirby - Volume 2 (Jimmy Olsen)
These two volumes collects Jack :King" Kirby's run on Jimmy Olsen in which he really cut loose!
Superman: Tales of the Bizarro World by Jerry Siegel
If you love the Bizarros and can't get enough of them, then you will want to get this.
Superman vs. The Flash
All the classic Superman/Flash Races in one handy TPB! This hasn't been release just yet as far as I know.
The Marvel/DC Collection - Crossover Classics, Vol. 1
Superman Vs Spiderman parts 1 and 2 and more stories with other characters, watch the Hulk get owned by Superman and more
Superman: Whatever Happened to the Man of Tomorrow?
Alan Moore official unofficial big send off to the Classic Superman, these were the last re-boot Superman comics made.
Belive it or not there are a lot more