Here's what I've learned in 5 years of having gear made for my motorcycle clubs:
Hats - the most popular, comfortable, and best looking hats are the Flex-Fit hats. They're like fitted caps, but they stretch a bit. They come in 2 or 3 sizes, so it's not like a regular fitted cap with a million sizes to get stuck with.
T-shirts - go with Hanes Beefy-T's. Best quality and a nice thick t-shirt that'll hold up.
Hoodies - Hanes Comfort Blend. nice heavy material, yet soft and comfortable. And please God, no Mr. Rogers zippers; just regular front pocket hoodies.
Go light on ordering medium and small stuff; large, x-large and 2x are going to sell more.