You can say either was or is. (And I will also explain what it means when you say had eaten.)
That was the best food I have ever eaten.
Using was sounds natural here, and it belongs to your description of your visit to the restaurant, which occurred in the past. In other words, using was fits the narrative perfectly, especially in terms of flow: it uses the same simple past tense that all the rest of the verbs in the narrative use. Unless you tell us otherwise, we assume that it is still the best food you have ever eaten.
If you say is, as in
That is the best food I have ever eaten.
This is also fine. You need to note that the present tense is does not match the tense of the rest of the story (of your past visit to the restaurant). So it does not flow as naturally However, if you want to stress that the food you ate at that particular visit is still the best food you've eaten when you say your sentence, then you use is.
As for using had, as in
That was the best food I had ever eaten.
This is also fine. But now you imply that it was the best food you had eaten at the time you ate that food, but that you have eaten food since then that has tasted better. It was the best food you had eaten at the time you ate it on your past trip to that restuarant; however, it is now no longer the best food you have eaten, because you have eaten food since that trip that was better.
As for using the verb to have instead of to eat, as in
That is/was the best food I have ever had.
This is okay in spoken English. But it is informal. See definition 4 (Oxford dictionary), so it is okay for the simple present tene and simple past tense.
However, many English speakers might avoid saying
That was the best food I had ever had.
because it sounds bad due to the double use of had and the alternative eaten is available.
I disagree with the answer by CookieMonster when it says
"If you're talking about the action of trying that dish for the first time as part of the list of things that happened with you during your stay there, then use was and shift the present perfect tense down to the past perfect one on the time line:
It was the best food I had ever had.
As a native speaker I can say that this is not true. I don't know why CookieMonster says this. Perhaps he/she doesn't have a good grasp on sequence of verbs. There is no reason to "shift the present perfect tense down to the past perfect" when you are narrating a past trip.