I would like others to tell me which is the best way to say that you are allergic to something.
I am allergic to fish
I have allergy to fish
I have an allergy to fish
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You can use any of these and you will be understood. The second one is not correct because there needs to be an article (allergy is a treated as a countable noun). The first and third are both correct and idiomatic.
You might try answering this kind of question by looking for examples online. Googling tells you the number of results. "I have an allergy" (1.6million), "I have allergy" (0.1 million) "I am allergic" (1.8million). Checking the hits for "I have allergy" show either non-native speakers, or phrases like "Should I have allergy shots". In this case google does a good job of identifying which phrases are idiomatic.