Here's why you're wrong:
1) Sannin is a tittle given by Hanzo of the Salamander to three Jonin who put up a good fight. Therefore, Tsunade was a jonin (rank) and sannin (tittle), same as Minato.
2) They were going to place Kakashi (a jonin you yourself listed) as the position of Hokage. You say Minato is better than Kakashi. Why can't he be a good Hokage?
3) He was taught by Jiraiya, a Sannin. Sakura and Shizune (examples you used) were taught by Tsuande, a Sannin. Minato clearly easily beats them, making Jiraiya the better sensei, and as a result, sannin. Naruto has easily surpassed Jiraiya, but has not surpassed Minato (not until he can use TBB, IMO). Therefore, J-man>Tsunade; Naruto>J-man; Minato>Naruto (for now); Minato>Tsuande. :|
4) Kakashi can easily beat both Sakura and Shizune (don't go into her strength; did you see how Kakashi handled her in the bell training at the begining of Shippuden, and that was without Kamui). Minato's clearly the better sensei.
5) You should stop being a Tsunade fan-boy.