What new Champ should I get

A little undecided who I should get out of the newer champions I don't own. I can only get 1 and I'm a little split

Illaoi - just came out and looks strong but she'll probably get a nerf soon anyways so might be better not to get into learning her just yet

Ekko - probably the only AP assassin tank, could get used to him if he gets used a lot more or use him against others if I can learn him

Bard - heard he wasn't that good on release on top of tough to learn, I'm more of a Leona player anyways but if he would be extremely helpful I could get used to it

Kalista - I see her a lot on AI games getting 25 kills easy. Looks hard to play as with her jump but is she really that strong in most games that I should have her in case I need to get her early?

Just wanted to ask which is more important in everyone elses opinion