60 deg, slightly overcast, downhill with the wind at my back, brand new paved road with no cars and a gaggle of hot babes waiting at the next stop with cold beer and a massage...ect.
Don't know, those sound like pretty good conditions to me!
I have rode my bike when it was 20 degrees, and when it was over 100. Worked headwinds and hills, occasionally with groups, but usually alone. The conditions for me that make a great ride are those that allow me to excape the mundane duties that I owe to my family and vocatoin! I'll take about any reasonable atmospheric or road conditions, as long as I get to ride!