Learn English – Opposite of “trendy” without a negative meaning

Let's start here.

intense: 1. (of a condition, quality, feeling, etc.) Existing in a high degree; forceful or extreme: "this job demands intense concentration".

So, given that. I'd say that whether it is positive depends on the context.

Wow, carrying these boulders is intense.

Would be negative (assuming you don't like to carry boulders).

The football game was intense.

Could be considered positive.

Preamble on the logical implication of an antonym

You are trying to confuse people's understanding of the term antonym.

Logically, speaking, the term you could have used is antithesis. However, since all the dictionaries define antithesis as exact/direct opposite, you should use the mathematically more precise term of null-hypothesis

The antonym of good = bad
The null-hypothesis of good = not good <= {bad, not bad but not good}

For the hypothesis
John = good person

then the null-hypothesis of the correlation is
John = anything but good.

In English, it is very frequent for an antonym not being the exact null-hypothesis. In fact, it is frequently true that almost all natural languages have antonyms not being the null-hypothesis. This is due to the absence of a pure binary set of possible states in such cases.

If a phenomenon Z has the possible states sorted by intensity {a, b, c, d}, then we can be sure that

antonym of a = d

But, the null-hypothesis of a = not a = {b, c, d}

What is more difficult when the states of a phenomenon is unsortable or unrankable, then any state within such a phenomenon does not have a generally acceptable antonym.

Therefore, the antonym

Therefore the antonym of the word prefer is indeed among words describing its extreme opposite

• disapprove, disfavour, dislike.

You are asking for the null-hypothesis of prefer, and as I have anecdotally stated, the null-hypothesis of many words is not the generally accepted antonym, due to having non-binary repertoire of states. Prefer is one of those words.