The first one (using was) is completely correct the other one can not be used since you are not telling the story in the past.
No matter what you say, I assure you I think that the evening we spent
together was fantastic.(Correct)
You are telling to a person that (yesterday) was a fantastic evening, this occurred in the past and the action is over. It does not mean however that you won't have another fantastic evening.
No matter what you say, I assure you I think that the evening we spent
together had been fantastic.(Incorrect)
You could use this only if you're talking about a past situation continuously. For example,
I was telling you yesterday that the evening we spent together had been fantastic (although was can be used as well)
OR
He told me that the evening we spent together had been fantastic.
Albeit, you can see that with using had been the meaning changes dramatically.
Had been is not really in usage. Try to avoid it when having conversations.