It all depends. Viktor and Zed share similarities in their zoning potential (if the enemy is bad), but they overall play far more differently. Kat is alike Zed in the sense that they both wait for their target to be really low and go in.
Viktor, like many mages, works best on waveclear and poking in laning phase. Your E and Q deal insane amounts of damage mid-game to late-game, and as a mage, Viktor scales far harder than Zed or Kat.
Zed, like many AD Assassins, works best on poking and roaming in laning phase. He does have a huge number of counters though (most of which are almost all the midlane mages), and doesn't scale as hard. It's better to have a scaling ADC on your team like Draven, Twitch, or Tristana, because if you can't end with Zed, you'll fall off, but your team can continue and you can provide support here and there.
Kat, like many AP Assassins, works best on poking and roaming in laning phase. She has some counters (most of which are probably mages), and scales harder than Zed, but scales less than say, Viktor or Syndra. She's adept at turning entire groups of squishies to lumps of flesh if you play her properly. Her AP advantage over Zed allows for armor to be useless, and forces people to build any MR. If they're autistic, they won't.
Zed and Kat are interesting. Although I've got most of Zed's core mechanics down (and I'm far from the best Zed NA), I've struggled with Kat properly. Weaving in the W as I jump from unit to unit is just... difficult for me. Zed has a kit that's easy to pick up (anyone can WEQ), but difficult to master. Not everyone can W+R+E+Flash+W+Q+R. It took me a while to get it down pat properly.
Kat's kit is similar, and a matchup between the two is a skill-matchup. The better player wins. I can do a basic Q+E+R+W, but anything higher than that and my mind explodes. Mostly because as Kat, I feel that I move much faster than I do as Zed-- consequently resulting in me not understanding what I'm doing. Idk. Maybe I haven't played enough Kat myself.
Logically, I'd suggest going with Viktor, as he does work better in team-fights, his W+R can change entire team-fights (like at Baron or Elder or any other contestable objective), and he also scales FAR harder than Zed and Kat. The only downside is your lack of mobility in comparison, but I'd think your increase in power over Zed and Kat works to your advantage.
But I'm an illogical individual, so I say Zed. He's easy enough to pick up, and mastering him is far more fun than Talon or Lux (from my experience). He works great in SoloQ, and since most ADCs played these days are hyperscalers, keeping your ADC safe while you take out their backline (or frontline) pays off. He's the Poster Boy of assassins, as he has the ideal kit (ranged dmg, melee dmg, mobility with a fairly high cooldown early game), as well as the Poster Boy of Outplays (look at the moves, Faker, what was that?).
Then again, it's up to you. I'd suggest trying each one out in a Bot Game (Int-ermediate, anyone?), and seeing which champion you feel more comfortable with. That's how I made up my mind between Ahri, Zed, and Twisted Fate. I kept running Bot Games of each one until I felt fluid with one of them, at which point I'd take them into actual PvP, which is how I fell in love with Zed (no homo). Ahri was fun ( ͡° ͜ʖ ͡°), felt too slow in comparison to Zed, and Twisted Fate, although he was the first champion I technically "mained", wasn't as fun.
... they r good and u can run down every elo!
