I hate to be the odd one out, but I think my least favourite crosses would be an arab cross. Mainly because any I have met are hot headed and not all that smart. Not to say there aren't some wonderful ones out there!!!
My favourite crosses would have to be appendix (QH x Thoroughbred). The ones I've worked with are smart and athletic. I also had a QH x Appy who was wonderful and taught me many many things over our 18 year relationship :)