I would agree that Tomahawk and Robotman are reasons to look for Star-Spangled, not the Robin cover feature.
I usually like the Mr. Terrific and Wildcat stories better than the Wonder Woman story in any issue of Sensation.
The Vision stories were usually the best part of the issues of Marvel Mystery
Any DC war book that had an Enemy Ace backup got purchased, period.
I'd say Tales of Asgard, except that the Thor feature was just as good!
I definitely also bought DC Comics Presents for the "Whatever Happened to..." feature, but I also bought Brave and the Bold for Nemesis.
I never really cared for the Scalphunter feature that took over Weird Western after Jonah Hex got his own title, but the backups were characters like Cinnamon and Bat Lash that I did like (and bought the book to read).
On a similar note, Warlord got same-old, same-old after a while, but I usually liked the backups, including the Omac one that seems to get forgotten when people discuss the character.
I also tended to buy the Dollar Comics like Batman Family and World's Finest more for the Creeper and Man-bat and Hawkman stories than the cover features.