The ones that touch your paint and the ones that stop cross-contamination. A quality wash mitt, a second bucket with a grit guard, and labelled buckets so wheel and paint gear never mix change your results more than most products do.

When they start working against you. A mitt that has hit the ground, a brush gone stiff, a sprayer that leaks or a foam pad that has torn all belong in the bin. Worn gear drags grit and undoes the careful work everywhere else.