The word you want is "argumentative," not "arguable." When you don't agree with something, but do understand the reasoning, you call that an "arguable" point. For example :) "My wife says I am an argumentative person, and arguably she is right." "Argumentative" means someone who enjoys arguing--enjoys the verbal game of debating with somebody. It is a slightly critical, negative thing to say about somebody. "Nitpicker" is informal. It's negative. It literally is a reference to looking for "nits," louse eggs, in hair. Nits are very tiny and difficult to see. In order to "pick nits" you need to look through someone's hair slowly and carefully, spotting the tiny nits. A related expression--with the same literal reference--is "to go through something with a fine-toothed comb." A nitpicker isn't necessarily someone who enjoys arguing, but just someone who is very detail-oriented and won't accept anything that isn't precise. "Quibbler" is a perfectly good word to use. Another is "hairsplitter." A less negative one is "perfectionist."

"A nitpicker". This is a very casual term for someone who is very particular about anything and everything. You can also use it as a verb to say someone "nitpicks" every last detail.

Very fussy person who argues over (insignificant) details. If you want to be polite, you can say a person is very particular (over the details) OR pays great attention to details.

He is argumentative, quarrelsome, opinionated (having strong opinions), contentious. For some idioms： He's a devils advocate. This means someone always argues for a controversial opinion or always looks for the tiny problems in a solid argument. He has a chip in his shoulder. This means someone is in a bad mood and consequently, they are likely to argue about anything.

Thanks for Madeline, Kate, and Learner SC's answers! They've been really helpful for me! But in the end, I find Dan Smith's answer more rich and detail-oriented. So I chose him as him the best answer. I hope the others won't be disappointed! I love everyone's answers really! Wish you a happy day!