we're told to use present perfect when the time is not mentionedThey’re two completely different words, and what verb tense they’re used with depends entirely on what it is you want to say and in what context.
Last, in this usage, is an adjective:
the last movie I saw = the one I watched most recently
Best is a superlative (good—better—best):
the best movie I’ve ever seen = my favourite movie (as things stand at the moment)
the best movie I ever saw = the movie I preferred to any other (when I saw it)
Thank you very much . very clear answer.The (exact) time does not have to be explicitly mentioned. What needs to be clear to make the present perfect inappropriate is a reasonable assumption that the situation referred to occurred in the past and is not being related.
Luke: Why did you leave England?
Emma: Well, my wife and I divorced, and I thought I'd travel round Europe fand Asia for a time.
Luke and Emma clearly know that they are talking about a past-time situation.
Jane:Have you seen any movies recently?
Jeff: No. The last film I saw was Hacksaw Ridge. That was pretty good.
Jeff is locating the seeing of the film in past time. The last film that anyone saw is always in past time.