Lissandra is amazing! You just need a team that follows up. One of my favorite games I've played recently I was TF mid and we had a Lissandra top. Level 6 if i ever went top was a guaranteed kill because she could follow up 100% of the time and stun, which lead to my stun, then her root etc etc. Her early game kind sucks because she doesn't really do damage but once you get a couple items and get 40% cdr she's a monster. Shitty champion if nobody follows (so prolly shitty champion at lower leagues) but amazing if you're playing with some friends or just playing with people that follow.
Annnd before you ask Draegon...Ahri, Leona & Lux are all amazing so if you're thinking about buying them since they are going on sale...get them all!
Ahri is probably my 2nd or 3rd favorite mid in the game. Very mobile, an assassin, can roam and lane clear well. Lux has dropped off a bit but as long as you can land skill shots you can still almost 100-0 a lot of people once you get deathcap + void, so just farm farm farm til you get those items. If you get fed early your burst combo does ridiculous amounts of damage so just go gank bot and watch as your ultimate does 1/2 their life. Leona is my goto support. Really fun with another bursty adc like graves or draven. Your ultimate has a huuuge range and if you can land the stun great, but even the close portion of it allows you to EQ combo in to follow up.