I said Tsunade just because she's more useful to me outside of sannin teams than Oro or Jiraiya. Her team-heal can turn the tide by itself, and a cheap stun for 1 tai. Her self-heal is most useful when in conjunction with Oro's cursed seal, but it's also incredibly useful where the other sannins are worst - 1 on 1. Jiraiya is not very good 1v1, I'd say fairly poor, actually... Oro is also not terribly great because he requires 2 different chakra to do 35 damage.
It was a hard choice between Oro and Tsunade though... CS and Manda are amazing, and if you have lots of chakra his 35 damage attack is great too, especially since it destroys a chakra. I just think Tsunade is more useful overall, whereas Oro is usually better but is more team-specific.