Any day with dry air and sun is great, so long as it's a reasonable temperature (like above 5F). I like the dry fall days before the ground is sludgy; I also like roads with hard-packed snow rather than a sludgy or rutted mess. 25F, sunny, and dry is amazing.
When it's not unspeakably cold, it's the gloom that kills my motivation. Which is unfortunate, since many of my favorite cities/places I'd live if I could choose happen to be gloomy.