Actually, in this context, they are both wrong.
Because you are using a past tense, "He was the smartest person", one would assume you are asserting a fact that transpired in the past. You would, therefore, complete the sentence using past perfect: .."I had ever met."
However, it is quite common, in spoken communication to omit the perfect tense completely. This is grammatically incorrect but you your intention will easily be understood. But when writing, it is better to include the appropriate tense.
"He was the smart person I had ever met." - past perfect
"He is the smartest person I have ever met." - present perfect
I think the reason that a lot of people drop the perfect tense is because the use of contractions (though still grammatically correct) sounds like they have dropped them.
"He was the smartest person I'd ever met."
"He's the smartest person I've ever met."