No one game is essential to my design. I've voiced my disdain for hacks time and again, and I certainly don't make my game design life focused on redesigning the one game over and over. Anything I make might be a blend of components from three or four games, with an idea or two of my own thrown in for good measure.
Yet, despite this, I do admit that there are a few games that contribute their components more often than not.
Mage: the Ascension
Completely divorced from the clunky core mechanisms of the Storyteller system, the magic system of Mage is brilliant, but then again the game is called Mage, so it would want to be. Actually, there are a few great game ideas that set the tone for the various games in the World of Darkness once you strip them away from the core system. Those are the elements that give me inspiration.
Warhammer Fantasy Role-playing
The career system in WFRP has been a strong influence in many of my designs. The low magic grim fantasy has also been a strong aesthetic when I've produced non-modern settings.
The lifepath system in CP2020 has been a strong design inspiration in much of my work.
For most of the rest of my design practice, I believe it's important to experience and understand as many different designs as possible, see what they're trying to do, see what they actually manage to do, where they work, where they fail, where unexpected serendipity brings alternative effects to the table.