What an extremely difficult question...But also a fun topic! It depends on what I'm grilling as to what my favs are. And I guess for this discussion, I must leave out the WSM stuff which is a fairly long list for this gadget guy.
So, for the Kettle.....
1. Slow n Sear 2.0 with SS drip n griddle. It protects the porcelain bowl coating, provides two zone cooking and turns a regular kettle into a low and slow smoker. The pan directs airflow and allows for easy clean-up.
2. Maverick XR-50 thermometer with 4 probes. Allows me to monitor internal meat temps, grate temps and even ambient temp if I want. I can't believe I grilled for all these years without one.
3. Latest addition - the vortex. I've cooked wings, legs and thighs with this and have results that I've never achieved with regular baskets. Plus it gives me a reason to use the silly gourmet grate system which I really didn't use before. I'm sure many people cook amazing chicken other ways, but this is a fun, easy way to do it that works for me.
I've got many other items that I actually use more than these like charcoal tongs, chimney, hot gloves and probe thermometer that I really couldn't do without, but I consider those to be necessities and not really accessories.