I built a shed about 20 years ago using 6 x 6 untreated pine posts. For each post, I built a concrete pillar extending about 12 inches above the ground. At the center of each post, there was a ½” piece of re-bar extending about 4 inches above the pillar. On two sides of the post, there was a piece of iron with two holes in it extending about 6 to 8 inches above the pillar. Drilled a hole in the end of the post and set it over the pillar and then using lag bolts, fastened it to the iron bars in the pillar. This shed has seen 100 mph winds (Katrina) without any problems. It did have good bracing to the roof in all possible directions and the roof has over 2 foot overhang so the posts do not see much direct moisture. This is my preferred method.
My dad built a similar shed that lasted for about half a century before it was torn down for lack of use. It was still structurally strong when it was demolished.