If I try and conquer an unoccupied bf and the opponent plays Rengar Trophy Hunter to it, who's the attacker?

You're the Attacker. Your opponent is the Defender.

When you move your unit to the empty battlefield, you apply Contested status first — that makes you the Attacker (Rule 459.2.b.1 / FAQ #9820). When your opponent plays Rengar, Trophy Hunter to that already-contested battlefield, they enter as the Defender (Rule 459.2.b.2).

Rengar's special permission lets him Ambush to a battlefield with enemy units even if you have none there, but that doesn't change the designation — the player who moved first is always the Attacker.