what to buy...what to buy

Okay, so your options are as follows:

Zed Rengar Kha'zix

Technically Assassins:

Diana Ekko Yasuo

Rengar and Kha'zix take a lot of skill to play well outside of the jungle, and they're not great in all situations. In the jungle Rengar still takes a lot of skill to play properly, Kha'zix a bit, but less than Rengar.

Zed is the champion everyone hates, and to be honest he's not that great right now, he can get really strong really quickly, but falls off just as quickly in the late game. He's workable, but I don't like where he's at right now.

Diana is technically an assassin, in that she has a tremendous amount of burst damage, but she's labeled Mage/Fighter. This is because with her shield she's naturally quite tanky, and comes with a touch of crowd control. She's interesting and can be played in Top/Mid/Jungle, she's always at least relevant from what I've seen in terms of the meta shifts.

Ekko has been nerfed hard repeatedly due to knee jerk reactions, he wasn't nearly as strong as people thought he was, they just needed to learn to play against him properly, but because of his over-nerfed status, he's pretty easy to play against with just about any other champion. His mobility makes him a bit annoying, but his damage is extremely underwhelming if you don't let him stack his passive. He still works, he just has to be played perfectly to get his advantage running.

Yasuo, is assassin-like, he can run full crit/damage and just shred through people, or he can go with a fair amount of crit and build more damage, he'll be relevant most of the time either way, does great against tanks because with his ultimate active and a last whisper he has 85% armor penetration, he does great against squishies because he has a ton of damage. He's complex and takes a lot of mechanical skill to be played properly, but with some basic knowledge you can do quite well with him. Also if you ever get a Wukong or Malphite on your team it's practically a free win with all that knock up to run your ult off of.

Overall I think it'd suggest you either take Yasuo or Diana, they're more useful universally, and no longer top the ban charts any longer.