I think it's a tough call, but as of right now IT and Smart to start in the back-court with Bradley coming off the bench and playing IT-like starter minutes. You need one of Smart/Bradley on the court to handle Irving, and Thomas is hard to hide on a guy like Smith, but seeing as his role is to just shoot 3s IT might be able to handle it on a consistent basis. He certainly gives a lot of effort on D, it's just hard when your 5'9 and have weak defensive instincts.
Smart/Bradley/Crowder is easily the best defensive back-court we have, but this has been a problem for these guys since they came together. A lack of two-way players. That's why Bradley and Zeller have been so key, AB especially. He's the closest thing we have to a real true two way player. Think of how many Bradley pick and pops you've seen this season. Stevens leans in AB to play big minutes because he's a fairly reliable shooter while being a top-tier defender and no one else, save Smart when he's hot, can say that.
Bradley has to get out of this shooting slump. If he does, we need to start maybe Crowder, though I don't like IT and Turner together too much, and keep doing what we're doing. If not, IT should Start with Crowder, and put Sullinger and Zeller out there. Sully proved his worth last game, he needs to focus on boards and generating offense. KO gets big minutes even if he doesn't start, and save Jerebko for when you need a spark. Bradley is the key though.