The superlative is fine, but I would change the word order: "The Present Perfect is among the most difficult tenses for non-native speakers to learn."
Student or Learner
1)The Present Perfect is among the most difficult tenses to learn for non-native speakers.
2)The Present Simple is among the easiest tenses to learn for non-native speakers.
Is the superlative form correct in these examples or should I use the comparative form?
Your sentences are grammatical. The rule is simple and has no exceptions. If you're only comparing two things, use comparatives like more and easier. If you're comparing more than two, use superlatives like most and easiest.
Their new album is good. Their third album was better. Their first album was their best.