Since this site literally has the episode writers list available, I was able to see who wrote which episode. According to Wikipedia (and other sources, don't worry) the series began with Steven Banks and Jed Spigarn as the story editors for the series - meaning they wrote a few episodes themselves as well as helping the freelance writers with their episodes. The only JN movie writer who actually wrote for the series was Steve Oedekerk, who wrote "Time Is Money" (though John A. Davis apparently came up with the stories for a couple season 1 episodes). Jed Spigarn for whatever reason stopped being a main contributor to the series by the time of "Raise the Oozy Scab", after which he pretty much only wrote specials. Gene Grillo became a main writer by late season 1.
Steven Banks tended to write episodes including as many cast members as possible, usually including the school or Retroville as a whole ("Out, Darn Spotlight!", "Lights! Camera! Danger!", "Incredible Shrinking Town" being examples). He also wrote all Professor Calamitous episodes with the exceptions of "Great Egg Heist" and "When Nerds Collide", and didn't write many episodes early on but by the end of the series was by far the most prolific, writing the final TV movie as well. Trademarks of his episodes include Carl and Sheen's constant bickering and the endings are typically humorous at the expense of Jimmy (though there are some exceptions, such as "Best in Show"). He also wrote most episodes with horror themes.
Gene Grillo usually pushed boundaries with his episodes, writing the first entirely-out-of-Retroville episode ("Aughh! Wilderness"), the first space episode ("A Beautiful Mine") and the Jimmy/Timmy corssover episodes. He rarely used the school as a setting (With the exceptions of "Return of the Nanobots" and "How to Sink a Sub") and several of his early episodes did not feature Cindy, though by the end of the series he was a top advocate of J/C shipping ("Billion Dollar Boy", "The N Men", "King of Mars").
Jed Spigarn, rather oddly, chose not to feature Jimmy's parents much even in his early episodes though, as "Flippy" can testify, it's not because he was bad at writing them. He wrote the most "serious" feeling episodes ("The Eggpire Strikes Back", "Stranded", "Crouching Jimmy Hidden Sheen") and has an unusual fondness for Miss Fowl and Butch.
Those are just the writers with the most influence on the series, of course the others have their own style too. Anyone have a favorite?
Stranded touched me the most cuz of the amazing plotline, which is thanks to the wonderful writer who wrote it! I'm just a sucker for all those adorable J/C moments in it; I also loved the humor that was thrown in with Carl, Sheen, and Libby in the hovercar! 
Jed Spigarn was an amazing writer for it; hats off to him! Simply brilliant! 
