Harry Truman had ZERO executive experience (other than being a failed businessman) before he became VP. He was VP for a couple months and was totally in the dark before he became president. Truman is generally considered to be either the best or second best president of the 20th Century.
She is also the ONLY candidate among the four that actually has executive experience, and while you’re correct that Truman was one of the best, Palin’s experience is a close match with that of another good one, Teddy Roosevelt.