Huckleberries, hands down favorite. But I need to watch myself with them.
One time, back when I was just a young lad of 12 or maybe 13 years, I hiked the North Fork of the St. Joe from Heller Creek to Spruce Tree with a couple friends. Spent 4 days fishing and hiking, with the bonus of it was a banner, and I mean BANNER year for huckleberries. Bushes were just full of thumb nail sized berries, and they were all over. The three of us got in the habit of snapping off a branch and eating the berries while we hiked. Eat all the berries off the branch, reach down and snap off another. Easy peezy. Between the three of us, I'd be willing to bed we crushed a gallon a piece for the first couple days. Then it happened. We all were hit with the affliction that comes with too many berries - the trots. By the afternoon of day 3 there wasn't a piece of TP left in camp, and we were all sporting sleeveless t-shirts. Day 4 was our pick-up day, but for a good week following, all three of us had the foulest smelling gas, like push a buzzard off a gut pile type stench.
My friends went home, I got to climb into the car with my parents and sister, and make the 14 hour drive to eastern Montana to see my grandparents. Lucky them. My folks had AC, but much of the trip was spent with the windows down.