Nothing beats sitting with a customer and actually listening to them. I can't reinforce enough the importance of this. As companies grow, its easy to lean on reports / reporting to tell you "what the customer thinks & feels". But there is so much nuance in how people communicate -- and as humans, we're all naturally atuned to these signals. So as a baseline, I encourage all my teams to participate in live listening sessions, focus groups, sales pitches, etc at least 1x every quarter to keep their ear to the ground and stay close to the customer.
Of course, the above doesn't scale and we all have lots of things we need to do with our time. So I'm very interested in a few types of data to inform qual & quant: