Rengar is fotm, Lee probably isn't changing and Shaco has been the same for forever.
Honestly my favourite is Shaco he's the funniest imo. Run around control a clone and appear out of nowhere to shank people. Boxes are also hilarious just put them everywhere you think a enemy is going or will use as escape routes. Getting down his basic moveset is easy. Getting used to boxes, dodging and moving through walls with clones, and just controlling clones efficiently takes a bit of time. Wall jumps are basically the same as wall flashes. Oh and never run Flash its always Ignite for securing kills and smite (Q is a free escape on low cooldown).
Rengar is probably going to get more nerfs in the future he's been constant ban in a lot of streams I watch at higher elos. He's also kinda feast or famine. You get fed early and you will one shot all game. You don't get fed you'll try to one shot and die all game when you jump the carry.
Lee is also fun. He's just so mechanic dependent. You really have to get used to his ward jumps and insecs. It's easy to misjudge the Ult and actually help them escape or miss the follow up q. You also have to get used to the 2 abilities per key and when to use them. Not to mention his passive requiring 2 autos after every move for energy regen. This is a champion for those who want to dedicate to one pick and stick with it for a long period.