Which grading company should I use?
In short: It depends on the card and what you want from the slab. A pre-grade assessment names the company whose standard fits your card, and explains why.
Grading companies differ in how strictly they grade, how their slabs are received by buyers of a particular card, what they charge and how long they take. The right choice for a modern card you plan to sell can be different from the right choice for a vintage card you plan to keep.
I do not steer anyone to a particular company. The pre-grade recommendation is based on the card. If you then use the hub, you choose the company and I work within its submission terms.