This seems to deserve its own thread (specially if you're in the office and bored af).
Obviously this depends of your geo location and all that stuff but in my book a sunny 18⁰C day is the best. No sunburn, hopefully you don't sweat that much and depending of your inner thermostat you can either wear a hoodie or a t-shirt.
I have a soft spot for cold sunny days, like 2⁰ but with a lovely cloudless sky.