The most surprising start probably has to be the Bulls. I think the expectation was that they'd be a playoff team this year with all of their off-season moves, but it has translated into success quicker than I would've thought. DeMar is such an underrated player in the league.
The team underperforming is the Suns. Going from a finals appearance to a 1-3 start is not ideal. They still have the guys to be a contender in the West this year, I imagine they'll pick it up.
The team overperforming is the Warriors so far. Curry is just going crazy right now, it's not sustainable, the team isn't that good.