What's the best of the non novel Maggin stuff?
Lotta options there. Maggin’s easily top 5 Superman writers - I know many consider him the best period - and even outside Last Son of Krypton and Miracle Monday (or for that matter his other Superman prose pieces Luthor’s Gift and Starwinds Howl, or his excellent novelization of Kingdom Come), the guy churned out plenty of all-time great material.
The crowning jewel is The Living Legends of Superman in Superman #400, his take on how Superman’s legend endures in the mind of man over the course of centuries with an absolute murderer’s row of artists; if you only read one of his comics, make it this one. Must There Be A Superman? is another obvious one - it hasn’t aged spectacularly, but it’s still a seminal story for the character and genre as a whole. The Day The Cheering Stopped and The Greatest Green Lantern Of All were crucial to his framing of Superman in the larger DCU as a mythical figure even among superheroes, and he wrote a pair of the absolute best Lex stories of all time with The Luthor Nobody Knows and The Einstein Connection.
He wrote a ton of Superman comics besides, of varying degrees of quality; he was at the end of the day still writing mostly normal adventure stories for kids, and as such didn’t always get to flex his muscles here the way his fans know he could. Even those standard adventures often excelled though, enough to still stand out decades later; some of his more notable ‘throwaway’ comics less concerned with defining Superman forever so much as telling a nice little story would include Who Was That Dog I Saw You With Last Night? (a Krypto romance), I Flew With Superman (one of the best Earth-Prime stories cowritten with Cary Bates), I Can’t Go Home Again (a great tale of Clark Kent), and the original appearance of Superboy Prime. All of the stories mentioned here can be found on the invaluable Superman Through The Ages!, a site I’d recommend unreservedly to any fans of the character.