If I attack a battlefield and the enemy drops a Rengar as surprise defender and I stun it, does it still get the bf conquer if I don't kill it?

Yes, the opponent still gets the conquer.

Here's why: When you attack an empty battlefield, you become the Attacker. Your opponent drops Rengar as a surprise defender. You stun Rengar but don't kill it.

During Combat, Rengar is stunned so it contributes 0 Might to damage (Rule 423.1.b). But stunned units still survive — they need full lethal damage to be killed (Rule 423.1.c). Since you didn't kill Rengar, it remains on the battlefield.

After Combat Damage comes Combat Cleanup (Rule 461.1): all units heal, then Attackers are recalled if Defenders are still present (Rule 461.1.a.2). Rengar (Defender) is still there, stunned or not, so your attacker gets recalled to base.

Now Rengar is alone at the battlefield. Since you're gone, the opponent establishes control and conquers — scoring the point immediately.

Stun does not prevent conquest. As FAQ #8498 states: "A stunned unit conquers an empty battlefield. Stun only prevents dealing combat damage, but does not affect conquest of empty battlefields."