Hmm difficult.
Desert rain. I love the smell of rain.
When going shooting, the first round shot on a nice early cool morning.
Firing up a well used smoker or bbq.
Large walk in humidor with quality cigars.
Freshly cut or burning ceder wood.
Camping out in the mountains with pine trees, mountain stream etc away from everything. Specifically waking up after a night where it very lightly rained, stepping out of a tent and starting a ceder fire. The fresh mountain breeze, rain and burning ceder. That is my favorite but it certainly doesn't happen very often.
Thanksgiving would be an honorable mention.