I'm going to have to say Karin in this one. Tobirama isn't close by any stretch. Most of his sensing feats are overrated imo.
-Sensing Madara: He only did that after being told Madara was still alive. He didn't sense it upon being revive, in comparison to Minato who sensed Naruto and Kyuubi without anyone mentioning them
-Sensing Oro's cells: He wouldn't have sensed it if it weren't for Hashirama telling him about it first. He had no clue until Hashirama told him to inspect Oro more closely
-Sensing Karin's heritage: Uzumaki's have distinctive chakra as well as physical characteristics (red hair). Recognizing an Uzumaki should be fairly easy compared to others. Especially since Tobirama, being a Senju, had close relations with the Uzumaki. That's like giving him praising for sensing Sasuke being a Uchiha, a clan that he has dealt with his entire life
His finger detection isn't even better than Minato's. If you compare them both, Minato uses one finger as opposed to the two that Tobirama uses, and Minato can sense the same number of enemies in addition to determining them as shadow clones.
Karin is the best. Naruto is probably 2nd since I don't know anyone else that can sense evil thoughts in order to discern an enemy. After that it's probably a hyuga sense all seem to be pretty decent sensors, but Minato could be placed ahead or behind them depending on which feats matter to you the most, and then it's Tobirama although you could interchange he and Minato as well since Minato does have that setback of not being able to sense Obito.
Again just my opinion. Depending on how you give credit for certain feats and how much you hold certain failures against them, it can drastically change the viewpoint. It doesn't help that the few sensors we know about seem to be good at sensing for totally different reasons and from different perspectives.
