I know roans and Varnish horses are avoided normally. Same thing with Sabino.

What are common things people tend to not like with the horse's coat?

I want to breed nice rated horse's with nice coat's.

On the flip side what is the most coat color with high demand.

I feel like coat's are extremely subjective to both the individual person and the breed. So some coat patterns and colors are sought after in one breed while another thing is wanted in another breed. So I'd suggest figuring out what's popular/sought after in your breed and working towards that?

Well-rated Tobi in any breed is constanly wanted.

But as Tasi said it'll vary between breeds & people personal preference.