Sentence 1:Thank you for the most superb dinner that I’ve ever eaten.
Sentence 2:Thank you for the most superb dinner I’ve ever eaten
Which sentence is more natural to you?
Sentence 1:Thank you for the most superb dinner that I’ve ever eaten.
Sentence 2:Thank you for the most superb dinner I’ve ever eaten
Which sentence is more natural to you?
Although this is a matter of opinion, I believe that sentence 2, with the "that" omitted, is more natural to say than sentence 1.
The reason I believe is that if you look at literature, shorter sentences are used to "hype" up the reader, and for them to read faster. It's the same with sentences that you say, you want to convey to the listener how nice their meal was, and you want that to be as short and concise as possible.
In English, use: that/which/who when you're talking about the subject.
You can - but you don't have to - use: that/which/who when you're talking about the object.
I'm the person. I liked the cake.
I'm the person who/that liked the cake.
That's the cake. I liked it.
That's the cake I liked.
Therefore:
Thank you for the most superb dinner I’ve ever eaten!