Rek'sai and Gragas have always been strong
Garages for his dmg/tank/sustain/cc kit and Rek'sai for the same
Olaf just tends to do REALLY well with the juggernaut items (Titanic, Steraks, Deadmans, and Maw)
and Nidalee and Elise are those higher skillcap junglers who have been good for a long time (Nid since her rework)
I think Jax is doing really well in the juggler right now because of his tendency to do really well building attack speed and damage items because his cult passive gives him a huge amount of those offensive stats back as resistances, just need some health to top it off. He also does innately well against those Olaf's and Kindred's
I also think Yi is really strong right now with the Bloodrazor>BORTK>Steraks>Deaths Dance>PD build. It gives him a ton of dueling potential and the % hp dmg he deals from BORTK and Bloodrazor are turned into hp from the Deaths Dance passive.
Shyvanna does decently with similar builds but suffers from lack of mobility or reliable gap closers, and cc heavy laners/junglers are too heavily played right now for her to do really well
Vi could also do really well right now from Bloodrazor seeing as how her passive gives even more attack speed, % max hp physical dmg, and armor shred for that % dmg. But not building AD on her hugely detriments her combat stats. She would only be viable with Bloodrazor if you were facing against an extremely tanky team that lacked damage. Warrior jg item is just too weak right now compared to the other items.