Your native language, of course you will have to learn the grammar.
Body language comes most naturally, it's the easiest to read and it tells no lies.
The one you like more! ;-)
It depends on the languages that u speak.. for example if u are fluent in Hindi then Urdu will be so easy for u..if u are fluent in spanish u may find Porutguese as an easy language to master..... but still n my opinion English and Spanish are the easiest languages ever.... :)
Many people say it would be Esperanto and I agree with them