I would say a slant. We have used stock trailers, two horse straight loads and slants. We usually use a two horse straight load since we have to borrow trailers due to not having one of our own, but when we haul our horses to fair we use my uncles stock trailer that holds seven horses comfortably and they always travel at a slant no matter how we put them. They semm more comfortable that way. Monty Roberts and countless other people have done studies where they have said that slant load trailers are more comfortable ways for horses to travel.
Shaneequah, 1998 gaited Bashkir CurlyxArab mare
Johnny Cash, 2001 Arab x pony gelding
Brighty, 2013 Standard Donkey jack