Spring.  Wherever I go, I love spring the most.

The world is fresh again. 
The smell of damp loam.
The sound of water burbling down portages, cleaning the muck away & leaving hard ground to walk on.
The water is clear and clean and cold.
Campfires have purpose.  Firewood is plentifull.  Fire bans are exceedingly rare.
Trout are up top & hungry.
Crowds of fairweather campers have not arrived and left their messes behind.
A nice day is a beautiful day, all things relative to expectations.
Cooler on travel days.
A crappy day is another story to tell.  Who tells stories about the trip when everything was perfect?  Let me tell you about the time nothing happened...  Most stories I hear are about the hardships, the rain, the wind, the snow, the storms, the bragging rights stories.
Animals are out and hungry, though I often see more moose & bear on the highway.  More sightings.
Higher water levels, every step taken from the portage is +1 for me!
No bugs.

2nd fave is autumn for mostly the same reasons or their corollaries.

Summer is just too darn hot.  I do like swimming on a hot day, I'd just prefer the temp to be 5 - 10C cooler.
Winter needs $2k in additional equipment to be reasonable.  Hot tent, pulk, snowshoes, clothing.