From what I have heard, Lebanese Arabic is the closest, and still pretty far. Passable conversation is possible.
Maltese is definitely its own language. Arabic roots are there (theres a Semitic joke in there ) but it isn't arabic anymore. Its written left to right with a variant of the english alphabet.
Writing RTL or LTR and alphabet alone don't make a language different.
Hindi and Urdu are 90% the exact same language, and are mutually inteligible (Urdu speaker and Hindi speaker can have complete full conversation with each other) but each is written differently (one LTR the other RTL) and with different alphabets
See also Croatian, Serbian, and Bosnian. I also find Chinese to be interesting, e.g. Mandarin and (formal) Cantonese have a near identical written language, while the spoken language is completely different, views on whether or not those languages are different languages or dialects vary wildly.
In my books, the distinction between languages and dialects are so arbitrary that the best method is simply to ask the people that speak those languages/dialects. If they consider them to be different language (which Maltese speakers seemingly do) I call them different languages.
Mandarin-Cantonese is very interesting and a unique (to my knowledge) example where the same written language can be completely different to two different people.
I don't buy the argument of just asking the speakers. There are cultural, political, etc. reasons people may think things which don't conform with reality. Many Hindi-Urdu speakers get insulted by the reality that the languages are pretty much the same because they don't want to identify with people from another country their country is constantly at war with.
I know that the reverse understanding isn't too bad from chatting with a Saudi-born member of staff on holiday in Malta.
I don't think anyone would seriously consider it a dialect of Arabic though with its completely different alphabet and half the vocabulary and morphology coming from Italian languages/dialects, even if Malta hadn't spent the best part of a millennium trying very hard not to become part of the Arab world
How much do you consider Maltese its own language (as opposed to a dialect of Arabic)?