Corporal


 * Our Hero/B.J. Blazkowicz (Doom) (Generally assumed to be Corporal based on the novels, and that he was ordered to remain behind, he is promoted to Sarge if all secrets are found in Doom RPG)
 * Marine (Doom II)
 * Marine (Doom III)
 * Flynn Taggart
 * William Gates
 * Corporal Stout
 * Corporal Ryan
 * Adolph Hitler ("Austrian corporal")
 * John Grimm
 * Dean Portman

Behind the scenes
The hero of Doom 1-3, and RPG is a Corporal based various sources. Assuming no 'promotions or demotions' (beyond best score in Doom RPG). The main hero of at least one of the campaigns in Final Doom is Marine Commander implying a higher rank than Sargeant (Sarge).