Unseen Predator (P): Rengar's passive is simple. In bushes your auto converts into a "leap" which does nothing but give you range and a ferocity stack. Rengar gains 1 additional ferocity stack every time he uses an ability. At 4 ferocity his next ability is empowered. Empowered abilities give you crazy move speed, and bonus effects. Addtionally, Rengar has a mechanic called "Bonetooth Necklace" where it acts similarly to the "hunter" runes from domination. Every unique kill he gets gives him % bonus AD like a rabadons deathcap. Into Kha'Zix there's a unique interaction called "The Hunt" that gives you a 6th stack on the bonetooth necklace.

Savagery (Q): Rengar's Q is amazing. It feels amazing to use. It does a lot of funny number damage. It's an auto reset so I recommend using it directly after auto attacking to deal even more damage than before an AA. His Q is always a crit and scales with it, so I usually like to throw in a couple crit items, it also procs on hits, so Essence Reaver is amazing. Collector is also super good since it's lethality and crit. Your Q will one shot people really Early with these items. Now something people don't really seem to notice is the fact that Rengar Q actually gives you attack speed for the auto after it as well, so it's surprising for burst. This synergizes well with BOTRK, as an AA Q AA comes out really fast. Empowered Q isn't anything too special, just some bonus damage, and bonus attack speed afterwards.

Battle Roar (W): Rengar's W makes him super tanky, even without levels in it. If you take 1000 damage, and use W you restore half that. So 500 health restored. This doesn't have a long duration on the stored health though, so use it reactivly. The gray health doesn't go away after using an empowered W, so double tap it under certain circumstances to completely heal back all the damage you took in a fight. Against monsters you store 100%, so a double tap technically heals you for the damage you took more than what was inflicted. Empowered W just does bonus damage and CLEANSES CC this is OP into champs like Mordekaiser, as it can remove his ultimate entirely.

Bola Strike (E): Rengar E is usually just used to get another stack on your passive. It doesn't do a ton of damage, and it just slows a little bit for target access. Empowered E is useful if they're mobile since you gain speed, it roots, and then you can kill them. I'd rather use the empowerment on something more useful like W or Q. Not saying E is bad, just in my opinion Rengar's weakpoint. Still strong, just not amazing.

Thrill of the Hunt (R): This is most likely the most terrifying thing an ADC player will ever experience in their entire life. Your screen goes slightly dark, you hear the background music, and the claw marks appear. Then all in an instant your screen goes black and white with no trace of who did it. Rengar's ultimate gives him a free leap from anywhere he's standing. It reduces armor slightly which doesn't sound crazy but 24 armor ignored is 24 armor ignored. With enough lethality you essentially deal true damage to people with your R. You deal bonus damage and gain bonus movement speed for the duration of your ultimate.