I am a big fan of katarina since I purchased her in late august. The thing is as she has become more popular it gets harder to pick her and people are becoming more likely to think that I am just picking an easy champion. Are there any mid or jungle assassins that aren't really meta, burst mages are fine to. Thank you for listening
Diana can be extremely scary when fed because her burst is freaking insane on squishies, you can basically two-hit them into late game with Q + R or Q + R + R, yet she's very very risky because she's a feast or famine champion so if you get behind you're done for unless something amazing happens, still, she's not too difficult to pick up, learn a few tricks with her and beat whoever you're laning against and it'll be GG.
Zyra is underestimated a lot, especially in mid lane, however, if you can get maybe 2 AP items you can constantly zone with plants as they do a lot of damage whenever people try to go tank them and then if you can root a squishy while planting a seed (that slow is going to be a pain for them) into ulti into Q + another seed they will die in seconds because of how much burst damage Zyra can pull off including that there's no way to get away from it unless they have 
or 
which to be honest are never seen anywhere or they could have stupidly high mobility and SOMEHOW get away from you, but hey even there they might have to use flash so that's good for you.
She may seem like someone that wouldn't be an assassin but honestly she can be if she gets enough items, because if you can ult, and then go jump on whichever squishy you please, they will die in milliseconds from how much burst damage Quinn can put out, plus she's even better against people with bad reaction times because when you go out to jump on them they won't do anything to mess you up in time. An example of her burst are in these videos: https://www.youtube.com/watch?v=O6-o-Nt6gng https://www.youtube.com/watch?v=mVQdq_Ztg6c https://www.youtube.com/watch?v=wJBsD4cx1Ag
Yeah so those are my suggestions, if you do pick them up then hopefully you'll enjoy them and have fun. :D
Assassin 
hides in a bush. 
went over the river and through the woods and never made it out alive.
Brand is pretty unpopular and does a ton of burst if you play him correctly. I think Veigar and Syndra also falls into this categorization.
Talon is your more traditional assassin that isn't widely played.
viktor has great burst and a ridiculous poke, next to impossible to die in laning phase with him unless u royally fuck up
's secondary classification is assassin
he can be built ad or ap thanks to the voidlings, and he has an aoe silence that lasts 3 seconds at max rank
Best assassin
