I like hot. When I lived in Tucson, I would wait until the afternoon in the summer to go on my typically 3-4 hour rides. I had the trails to myself.
Optimal though, is probably upper 70s.
Not sure I've found my optimal in the velomobile. It seems to do better in slightly cooler weather (I assume because I'm warmer since it's mostly enclosed) and just recently set a personal record on one of my 15 mile routes home from work. In a ~70 lb machine with all my commuting gear, so closer to 80 lb, a light wind (mostly head wind, which barely affects the velo) and traffic, I still averaged 19.4 mph over 15 miles.