About 25 degrees celsius (the superior measurement, water freezes at 0 and boils at 100. Simple and easy). Not too hot or too cold, we get up to around 45 degrees max around here and that's hot, too hot. It never snows here, ever
In Fahrenheit I'd say between 55 and 65 degrees is my favorite (roughly 13 to 18 degrees Celsius). For me it comes down to a preference of being able to put on more clothes versus being unable to take off clothes. When you get to more extreme temperatures it bothers me more though. Really cold weather is bearable since I can put on more layers of clothes, but really hot weather just bothers me since you get to the point where taking off clothes would mean you're naked.
In between hot and cold, around 60 degrees Fahrenheit is all right for me, with a breeze. Cold and hot weather make it hard for me to breathe, and I don't like the snowstorms/thunderstorms that go more often with lower or higher temperatures.
60 - 65F is my favorite temperature here in Alaska. But I can be comfortable in any temperature as long as I am dressed properly for it. When I am in the lower 48, I love 75F. It's very comfortable and not too warm and not too cold!
Hmm... call me insane if you must, but I generally prefer the mid-to-late '40s Fahrenheit. Basically anywhere between 43° and 55° is great for me when I have a hoodie on (I wear a hoodie basically all the time during winter). For times other than winter, I really like the mid-to-late '50s (basically between 55° and 63°)
My ideal temperatures are between 15-25C, not too cold (while I'm used to cold, I'm really not fond of it) and at the same time not too hot (not too used to hot weather, so while I appreciate the higher temperatures it can get uncomfortable if it gets really, really hot).