Rook endings are the most common AND the most difficult so a study of rook endings seems to be top priority but NOT for beginners.
In "Silman's Complete Endgame Course", rook endgames (Lucena Position/Philidor Position) are to be studied when one is a Class C player (1400-1599). What's the probability of a beginner playing in a R+P vs. R+P endgame? I'm in one right now and it's insanely challenging!
I was speaking with a well known GM a few days ago and he said that he always taught his students the game from the end to the beginning. One reason he stated is because end game technique is concrete and will help your all around chess while opening theory is constantly changing.