No Prep Time
Setting Leaf Village
Seeing as Kakashi was able to instantly fully warp away BM Naruto, the fastest guy in the series, I'd say Kakashi.
He was only able to do that when Killer Bee, Gai, and Naruto were distracting Tobi so that Kakshi was able to fully focus on nothing but watching Naruto and making sure his attack would work. He wouldn't have 3 people distracting Tsunade in a 1vs1 fight so that he could focus all of his energy and concentration on landing one attack with no possibility of being attacked.
Having said that, I vote for Kakashi. Tsunade is a great medic but she's never shown even Jonin level combat skills. I think the best she's done is land some hits against Orochimaru when she outnumbered him 2:1 and he couldn't even move his arms.
@sickVisionz: Distractions had nothing to do with Kakashi being able to warp Naruto fully while already having used Kamui multiple times previously and other high-level techniques. He was able to keep track of RM Naruto and Tobi fighting against each other without anything distracting them and was able to threw a kunai between them before they even connected from a distance much higher than the distance between Tobi and Naruto before he threw that kunai. He was also able to react to BM Naruto rushing towards Tobi and no one even knew what Kakashi was planning. Tsunade is slower than Tobi and RM, there's no reason why Kakashi needs a distraction to be able to locate, react to, and warp Tsunade instantly (remember, he's able to do this instantly seeing as Tobi couldn't even see what happened to Naruto despite having a similar dimensional jutsu until he entered his dimension).
@phantomrant: Kakashi was able to do that when he positioned himself away from combat, had no risk of being attacked, and the enemy was distracted with someone else. He wouldn't have any of those advantages in a 1 vs 1 fight as the opponent would be trying to attack him, not letting him chill back in the cut so that he can focus on getting his timing right.