I voted for "I think the film's good". But I believe it is possible to use both case due "I think" is a present simple and "I am thinking" a present progressive. So "I think" it can be undestanding as an opinion, and "I am thinking" is an extended action.
I think something is good (now, it' my opinion about a fact)
I'm thinking I like a big icecream (I hesitate to do something in the near future)
I'm thinking it's no good idea (currentrly the accion no happened, it could happen in the future)