I smite a unit and opponent flips a zhonya, does the unit get saved by zhonya or is it banished by smite ?

Yes — the Zhonya's controller chooses, and if they apply Zhonya's first the unit is saved rather than banished. Zhonya's Hourglass ("If a friendly unit would die, kill this instead. Heal that unit, exhaust it, and recall it.") and Smite's "If it would die this turn, banish it instead" are both replacement effects competing for the same die event, and the controller of the affected unit picks the order (Rule 373, FAQ 10635).

If they order Zhonya's first, your Smite's damage still happens but the death is replaced by heal/exhaust/recall — the "would die" event no longer exists, so the banish replacement has nothing to modify. If they somehow ordered Smite's banish first, the unit leaves to Banishment instead and Zhonya's stays. Note this only works because it's their friendly unit: Zhonya's can only protect a unit its controller controls, so it can't save a unit you own.