Hmm.
That's a toss-up. I love making monsters. Monsters are what got me interested in RPGs to start with.
I also love to make classes. My witch is my foremost example, but I have many more.
But in the end, I would have to say spells.
According to my spreadsheet, I have written over 800 unique spells for various games.
Spells and magic are my favorites.