This was by far one of the easiest questions for me to answer. You'd think it would be difficult due to the sheer number of RPG-related accessories that exist out there, but as soon as I started thinking about it, only one thing popped into my mind and refused to go away. My choice is the GM Screen.
I know some people despise using screens and would probably set fire to every one they come across if they could. First, I think they should calm down and seek help for their clearly pyromaniac ways. Second, the presence of a GM screen at the table just feels right to me and every time I haven't used one, I've felt weird and exposed. I like being able to sit behind this large screen, hiding my notes and my rolls from my players,while also having easy access to some of the miscellaneous rules and tables right on the back of the screen, making the rulings on the flight much easier to accomplish.
Also, I would be lying if I said I didn't like the sinister nature that is accompanied by me hiding half my face behind the screen, rolling a few dice for reasons unknown to the players, and saying, "Interesting..." That stuff just makes me happy as a GM.