Both ”Jeg synes” and ”Jeg tror” are translated to ”I think”. But they are not interchangeable.
”synes” is very subjective and is only used when expressing a personal meaning that has nothing to do with facts. ”tror” does also refer to the speaker’s thoughts, but it is used when the speaker is talking about facts.
”Jeg synes det er best å ta bussen” means ”I think it’s best to take the bus”. By saying this, you only tell the listener about your experiences with the possible transport means. There are probably many other ways to get there and some of the other ways might be faster or more comfortable than the bus, but in this case the speaker still prefers the bus. This sentence may also mean ”I prefer taking the bus” or ”I like to get there by bus”.
”Jeg tror det er best å ta bussen” does also mean ”I think it’s best to take the bus”. But in this case, you tell the listener that you think the bus is faster or more comfortable than the other transport means. You don’t tell the listener what you prefer, you only tell about what transport means you think would be the best for the person that is going to travel. If the person that is going to travel is the listener, this sentence might function as an advice for him to take the bus.
:)