Hashirama- yea I agree she'd definitely lose no matter which way you slice it, hell he could probably sit there and tank her punches in sage mode if he wanted to =P (mabye not actually but you get the point)
Tobirama- We haven't really seen all of his capabilities but based on what we have seen I wouldn't say Tsunade>Tobirama so quickly. He has the bringer of darkness technique which would be a hard counter to Tsunade's very linear taijutsu style. Next up he has a huge, and very powerful, chackra-he essentially made the 3rd and fourth hokage, juugo, suigetsu, sasuke, and orochimaru scared into battle readiness simply by raising his finger, and none of them are sensor types or are known for being able sense others' chakra. So that counts for something. He also had the flying thunder god slash, which was fast enough to kill Izuna Uchiha, who had MS, from this we can take that at the very least he has 1 attack that tsunade likely can't effectively counter except with her Creation Rebirth technique. Also Tobirama is known as the greatest user of water style jutsu to ever live and we really haven't seen any of that. For these reasons and the fact that the ninja of the past (Madara, Hashi, etc) are made out to be stronger and he's made out to be #3 in the pecking order, I would not be so quick to dismiss him against tsunade.
Hiruzen-Pretty hard to tell with this one really. Hiruzen trained her so he'd have extensive knowledge of her skills and techniques, and also he knows every jutsu in the leaf that is not based around a kekkei genkai. Also for those of you who say these are all c-rank techniques and useless, remember its who is using them (Shikamaru is a prime example) and they are largely the lower levels of most clan's techniques that we've seen, so he probably knows the B,A, and S rank versions of many of these techniques. He also has enma, who is a fairly powerful summon. However we haven't seen most of these in action, so until we do, Tsunade has the edge but again I wouldn't count Hiruzen down and out until we see him in upcoming chapters as an edo to see his techniques.
Minato-Actually a decent argument was made for Minato above by pandu but I'll just add a few things. He made A in lightning armor and B, a future kage and a perfect Jinchuuriki, look hilariously under-powered and slow using only taijutsu and FTG (Also I know its in the anime but A is shown losing an arm wrestle to A b/c of his speed/reaction time and Minato is faster and has bttr time than A as a comparison to speed if you want to count it). If the fight would've gone on its clear who would've had the edge. Also where are you getting he lost to Obito? As I recall he figured out how kamui worked (on a practical basis) after seeing it once and countered it. To date he's the only we've seen do that (only once being the key word here) and make obito run for the hills in his "more adult" (there's disputes as to the time frame on a couple of threads I've read), again the only person to date to do that. If you count him being hit by Kurama as a loss, remember he was hit on purpose (and I believe that's the only time we've seen him hit by anyone) and he didn't have to lose that fight, if you want I can post the page. Lastly he's also the only one any village has put flee on sight orders out for, kage don't even have that (granted hopefully there's some common sense involved on the opposing sides, such as Gennin probably shouldn't fight a kage but jounin might). Due to the very hard counter Minato is style wise to Tsunade I have to say Minato>Tsunade in a straight fight, unfortunately even if you have a 1 hit KO its no good if you can't hit him unless he wants you to.
So yea that's my thoughts, she's not the strongest and likely not the weakest but until we see more it's hard to tell =) good thread though. Lastly as far as the Sannin go my two cents is Jiraiya>Oro>Tsunade but again that's just an opinion =P I love the pervy sage =)