I'd say persian is similar to japanese. the grammatical order is identical (subject, object, verb), except the particle order is reversed (particle noun, instead of noun particle), and the words that are irregular in japanese are also irregular in persian. but persian's also similar to english in that different conjugations are required for different sentence structures (adding "if" or "can" or "know" or "think" to a verb requires conjugation in persian, but not in japanese).

Persian has basically no gender, which is close to English, as well as similar tenses(progressive, perfect, simple/past, present and future). It has six persons' verb ending, which resembles Spanish, French and other languages. Persian contains about 30% Arabic loan words, and the writing system is the same as Arabic. It's a typical Indo-European language, Tajik can be considered as derived from Persian.

I think Tajik is more similar to Persian because Tajik is just like Persian and it's difference is in pronounsation.

well, most of ppl think that Arabic is more similar to Farsi. but it has some difference with it (not much).