Gangplank is far from bad after the Triforce nerfs.
No offense meant, but that most likely means you just aren't a good Gangplank player.
I'd recommend you to watch Tobias Fate's Stream. He's a challenger Gangplank onetrick.
Here's a montage he recently uploaded to his Youtube channel.
https://www.youtube.com/watch?v=ZAphNVtNmmc
Just check him out and see if you like his content.
-_-
As far as Fiora vs Renekton goes, Renekton is a champion who requires game knowledge and Fury management. He's a lane bully who gets a lead and snowballs the fuck out of it.
Fiora is a champion who is arguably a bit harder than Renekton.
Knowing when to use her W is the hardest thing here. She scales a lot better than Renekton does, so doing bad in the laningphase is a lot less punishing than it is on Renekton.
Hecarim is a champion who is reasonably easy to pick up, but is pretty hard to master.
He's currently pretty meta, and might get nerfed soon.
He's a true diving fighter.
He goes in and tries to kill atleast one carry before he dies (or the enemie team dies.)
He has his E and R for mobilitly, but uses both to get into the fight.