This is a tough one.
Roto - I love it and rarely do whole chickens without out. It is probably the least used of my accessories, but I am glad to have it when it gets called up.
Vortex - If I leave it at a friends place after doing some wings on a game day I will still make wings at home with my charcoal baskets, but if the vortex is here it gets used every time for wings. I also use it quite a bit for reverse searing, but not as much since I got my slow-n-sear.
Kettle Pizza - I wouldn't even think about not using this. If it was taken from me for some weird reason I would probably just not make any pizzas on the grill until I could replace it. This accessory will result in pizza-itis similar to webitis. You will go through a lot of charcoal and wood when using the kettle pizza.
Slow-n-sear - Stays in my grill for most cooks. I use it for high heat cooks, reverse searing, and slow and low cooking. I move it from my 22 to my 26 if more space is needed. Can be used with a roto.
So which one is a must have....I'm not even sure. I'd say the slow-n-sear has more uses than the other options and gives you more cooking space, especially if you are currently using the snake method for slow-n-low cooking. I would say that you probably wouldn't regret having any of the above accessories.