When I bought my grill I spent way too much time debating between the available accessories. After six months of having my grill, I've determined the following:
Basket Splitter
Cover
Side Tables WITH Stainless Tops (Best Upgrade IMO)
I purchased the baking stone, rotisserie cradle & reducer, and the cold smoker when I ordered my grill. Sadly, I have not used the cold smoker or rotisserie yet. I've used the baking stone a few times which was quite nice but not an essential.