Please. You're being dump. Shiki-Fuujin was used to seal a tailed beast. You're actually the one who truly require some common sense, as I see it. Each jutsu, sealing jutsu or not has a purpose. The specific purpose of shiki fuujin was to suppress and seal the power of nine tails within the death god. True, it was his strongest jutsu for that specific purpose, and that doesn't mean "ANYTHING". He knew seals for a variety of purposes, if you wanna say, like the contract seal which he used on Obito. It was entirely different technique.
There's no point comparing a jutsu with another - Each a have a purpose. They exist to serve their purpose. A strongest jutsu for a purpose exist. There is no overall most powerful ninjutsu or genjutsu OR fuin jutsu.
Besides, you say this is common sense. If you really had this so called 'common sense' you would have speculated that Minato had a variety of techniques in his arsenal. He had every technique he needed for every situation he found himself in. One could argue that it was a plot no jutsu. But no, "he knew enough techniques that he could counter any situation he found himself" would be the most logical solution according to common sense. Honestly, if you wanna state otherwise then you'd have to say that all jutsu's Minato had was Tobi/Kyuubi specific. That would make you nothing more than a Minato hater.
Then, you said he didn't have a jutsu stronger than Rasengan. If you really believe that someone is made kage with only one offensive jutsu, then you're the actual bust. The reason he used Rasengan on Tobi, all according to common sense, is because since Rasengan doesn't need hand signs. (Remember, minato said that the one who lands the first hit would have prevailed in that battle) Which was also the reason that Obito didn't use another jutsu rather that his bare hands. He didn't have the luxury of time. That was the true purpose of Rasengan, a jutsu that didn't need any hand seals.
Minato did everything for a reason. Which was why he invented Rasengan, a technique that goes along with his speed advantage. That is where his true genius lies. Rasengan is a technique which complemented Minato's speed more than anything else. He could use Rasengan at a situation where Obito could NOT use any other jutsu. So the conclusion is that he used Rasengan because that was the best choice in the situation, one where he did not have to take chances.
There is a lot more of Minato's techniques to be seen. That far, is clear. What all those techniques are we do not knew. He intended to combine Rasengan with his affinity, which would mean existence of least one natural affinity. Personally, I believe he had more than 2 seeing he was a Kage. This would naturally lead to another logical conclusion that he knew other techniques which made use of his element releases.
All according to mere common sense too.
