I prefer it to be warm to ride
My horse colics from the heat here in South Africa
It depends on how cold/hot... it gets pretty terrible where I live in the winter so I would say hot for me!! My horse also prefers the hot weather, makes her feel a little better!
Cold definitely! Especially if you have the advantage of an indoor. In South Africa riding in 40+ weather and humidity all summer is horrible!
I consider hot weather to ride in
On the colder side, but dry. Unfortunately, I live in a very wet country, so good waterproof gear will have to suffice.