Laptop is a countable noun and I understand it is a general meaning here. I know it's correct to say it like this and it feels natural. But why is it so exactly? Why is there zero article?
What is a determiner in that sentence. The fact that laptop is a countable noun doesn't matter - you'd also ask what dog do you have?.
Articles are also determiners, and you don't generally use two determiners for the same noun (there are some exceptions, as @NigelJ mentions). So when you use what, you don't add an article.
For more information on determiners, see for instance wikipedia.
What (or which) can of course also be an interrogative pronoun, but here it is an interrogative determinatives (functioning as an interrogative determiner). In a sentence like
What laptop is the best laptop?
Which is a determiner for the first occurrence of laptop, and the for the second occurrence of laptop. Of course, we usually omit one of the two laptops in such a sentence, but the determiners remain:
Which laptop is the best?
Which is the best laptop?