Disclaimer number 1: I have no experience with Yasuo and little experience with Zed.
Disclaimer number 2: This is my biased opinion.
1 - Kled is insanely fun. Of all of them, I believe is the one with the biggest adrenalin rush moments. When you lose Skaarl in a team fight and quickly recovers you fury only to have the the glory of recovering that goddamn lizard and completely turn the tides of a battle... Ohh, there is no describing that feeling. He's also sort of a lane bully if you are good with him. Bonus points for only needing 1 AA and 1 Q to proc thunderlords. They never see that burst coming. One of the funniest champions I've played lately.
PS: CHAAAAAAAARGE!!! It has 8 A's. No more, no less.
2 - Talon, when fed is a goddamn beast. If you ever get an edge in lane you can constantly all in your enemy, farm a bit, recall, all in again and get another kill and repeat this forever. His ult CD is laughably low, so as far as you're ahead the enemy laner will not know the meaning of "breathing room". If you get behind you're still useful as all of your burst is AoE. You will most likely get deleted in team fights unless you're ahead, but you deal enough damage to make up for it. You're also a beast late game with the fastest burst in the game.
3 - Rengar is the bad pussy you need (buy him now if you wanna make stupid GoT references like this one). He is great in solo Q, where is each man for himself. With DQ rolling around, he suffers a bit in higher ELOs, but he can still cause mayhem and get extremely fed easily. Also, his counter jungle cheese comes close to Shaco-ish levels. When fed, he turns the game into a horror survivor for the enemy team. When not fed he's just a harmless kitty unless he can set up plays with his empowered eBola.
4 - Zed is the one I played the least. The reason is simple: you are as reliant on your ult as it gets. It feels great to one shot players in the mid and possibly late game, but he just feels weak without his ult up. He's the only one in this list I never had fun playing. I can't personally recomend. Seeing how gutted he is and how often he's banned, I wouldn't invest my IP on him just yet. Maybe wait until after the Assassin Update, see how he turns out, wait for his ban rate to decrease and then make up your mind.
Never played Yasuo, so I can't recomend him.
Honorable Mention - My boy Shaco is a thing of his own. There is no single champion I enjoy more than him and he's probably the reason I still play League. He has a simple kit, but mastering him requires much more Game Knowledge than Mechanic Skill, although he can bannefit a lot from it with all his shenanigans (Using your ult to go over walls and dodge abilities, blocking skill shots with your box and that's basicly it). He requires snowballing to be effective and against coordinated teams he suffer a lot in team fights, but he has all the tools one might desire to snowball himself and his team pretty hard. He requires a lot of pratice to be truly useful, but I think it's one of the most rewarding and tilt inducing champion when played correctly. Playing him feels more like a battle of wits than a mechanical contest. If you want to be an annoying motherfucker, pick Shaco