Present perfect or past simple - "the best movie I (saw/'ve seen) this year"

Solution 1:

What is often taught as a "rule", that present perfect cannot be used with a specific time, is only approximate. The present perfect is a present tense—it denotes a present state arising out of a past eventuality—so in practise it is not used with an expressions designating times which do not include the present, the time of utterance.

This year, this month, this week, today and similar expressions therefore work just dandy with the present perfect, because the timeframes they designate include the present—they stretch up to and include the time of utterance (and, usually times beyond the time of utterance: I intend to finish this answer tonight).

They also work just dandy with the simple past, because they designate timeframes which include the past time during which the eventuality was realized.

As for your specific example: it depends on the context and on what exactly you mean. If, as deadrat assumes, you mean this movie is the best so far this year, then Standard English (whatever that is) prefers that you employ the present perfect.

Colloquial English, however, particularly in the US, is not so picky. Nobody I know would sneer at you if you said "It's the best movie I saw this year". Almost nobody I know would even notice.

And it may be that your context and your meaning are something different. For instance, if it's after Christmas and you're a film critic preparing your annual review, with no intention of going to another movie until the new year, then it's entirely proper, in any register, to say "It's the best movie I saw this year."