This is a message to all thread starters: NEVER compare Minato to people other than the following:
Hashirama
Tobi
Madara
Hiruzen
Tobirama
3rd Raikage
Muu (maybe)
Ooniki (maybe)
2nd Mizukage
Itachi
Nagato
Kisame (maybe)
Jiraiya
Orochimaru
Dont bother with the rest as he'll clearly win!! His speed and intelligence makes him VIRTUALLY undefeatable...
Remember: The last person, beside Naruto, to ride on Gamabunta's head was the Fourth, Anko wishes the Fourth was alive when Orochimaru attacked Konoha, Tobi says Minato deserves the title of Fourth Hokage, 4th Raikage believed "There was never a man that could surpass him". He makes everyone (apart from people on the list) look like a bunch of amateurs...
SIDE NOTE: I believe Minato can go into Sage Mode. If he has signed a contract and can summon frogs, surely a man of his intelligence would've wondered where they came from? Im guessing he asked, and they took him to the mountain and explained everything to him. Jiraiya states that Minato's "guts and desire to succeed were the fiercest I'd ever seen". I dont think he would've given up on senjutsu and would've mastered it (atleast to Jiraiya's level).
To those who say 'Why didnt Minato use it against Tobi?' Well, Im guessin Minato uses the FTG-Rasengan combo to test out the opponent's strengths and weaknesses. If things get REALLLLLLY bad, he'll probs use Sage Mode. Also, after the Rasengan struck, Tobi left and Minato had achieved his objective: detach Kyuubi from Tobi.
Even after fighting Tobi, Minato had enough chakra to use space-time barrier on a frickin Bijuu ball, summon Gamabunta (which takes enormous chakra, as stated by Jiraiya), and then teleport himself and the MASSIVE Kyuubi away from Konoha, followed by the use of the Dead Demon Seal. Id say he has enough chakra to be able to use Sage Mode, as Sage Mode can only be used with people who possess huge amounts of chakra.
Then again, giving Minato SM would just make him even more powerful, maybe too powerful, therefore Kishi may not give it to him. I personally think he's got it, and we should be treated to a fight in which he uses it.