If you plan on playing ranked or unranked, don't buy operators that get banned alot. Jackal is good on attack. He can track footprints but he does get banned. Depending on your playstyle, that's who you should get.
I started playing Rainbow Six Siege this week,i want to know what characters should i buy first,i have bought the deluxe edition so i have the year 1 and 2 operators,and i feel that i lack intel on attack,like knowing where people is,so i was thinking on getting Iana or Zero.