What do you mean by strongest? In terms sheer of power? I would say Gaara then judging by the massive jutsu he's done so far Kitsuchi. If they were to fight I'd have to say Kakashi takes it to gaara and kitsuchi. Apart from the obvious Kamui, his lighting techs match up great to Gaara and Kitsuchi's sand and earth techs respectively. Also Gaara and Kitsuchi are long ranged fighters and with Kakashi's speed, experience and sharingan he can match jutsu tit for tat, till he's in close enough range for a kill strike. Darui's black lighting tech's would be a good match up as well but he'd still have to get close enough for a kill shot and though he'd be a good fight with Kakashi I think the Sharingan and Kakashi's experience would be the deciding factor in that match up. All we've seen from Mifune suggest's he's a straight up close range fighter, but I don't think he'd have enough power to penetrate Gaara or kitsuchi's possible defenses. And against Kakashi and Darui all he really brings to the table is a bad ass knife to ninjutsu fight.
In terms of Power
1 Gaara
2 Kitsuchi
3 Kakashi
4 Darui
5 Mifune
VS
1 Kakashi
2 Gaara
3 Kitsuchi
4 Darui
5 Mifune