There are 3 heavy hitters in my book; Spy Smasher, The Masked Marvel, and King of the Rocketmen. So lets start with these!
KING OF THE ROCKETMEN
Tristram Coffin as Jeff King/Rocket Man
Mae Clarke as Glenda Thomas
Don Haggerty as Tony Dirken
House Peters Jr as Burt Winslow
James Craven as Professor Millard
I. Stanford Jolley as Professor Bryant
Stanley Price as Gunther Von Strum
King of the Rocketmen is a misleading title, because there are no other rocketmen in the entire 12 chapter serial. The title is as such because of the hero's name, Jeff King, played by Tristam Coffin, who for once is not playing the villain. The villain, Doctor Vulcan, needs to kill the world's leading scientists (who house themselves at Science Associates) before taking over the world. His 1st target Dr. Millard, who narrowly escapes death, lives secretly in a cave while his friend, Jeff King ala jetpack, tries to uncover who Doctor Vulcan really is. All the while, trying to keep his identity as Rocketman a secret. All this, in chapter one.
I love Rocketman. It's perfectly paced, I feel like Republic serials tend to be (but I may be a little biased, since Republic is in my top 3). The 'dame', Glenda Thomas, is not your typical damsel in distress and not seeing the villain keeps you on your toes. It's awesome seeing the Rocket suit in action and the sound effects before take off is one of my earliest memories. This was the 1st time the Rocketsuit was introduced, and it was seen in several serials after.
And flying with a jet pack... is just plain cool.