When you use a quotative-と, the "quote" will basically be a normal non-polite sentence. That is, you can safely use は inside the quote, and you should not drop だ. Brackets are usually not used in a simple case like this. Therefore, the correct sentence is:
The polite version is:
As you may already know, 私は is normally omitted, so the following is almost always more natural unless you have to emphasize "I":
彼はいい人だと思う。 / 彼はいい人だと思います。
Note that this does not mean something like "He thinks (about) a good person". See this question for the reason.
There are exceptional cases regarding と:
- だ is sometimes dropped in informal sentences (known as だ抜き). 私は彼はいい人と思う is acceptable in a casual conversation.
A + を + B + と + verb is a different construction from this, and you can safely drop だ in such cases.
These are discussed in this question, but if you are a beginner, you may want to learn about them later.