I'd wait with Aatrox currently. Nobody has any idea how he will turn out after rework, check this post saying that it was/is already in testing at least partially
https://boards.na.leagueoflegends.com/en/c/developer-corner/KtbGvapw-patch-chat-with-the-playtest-team-724?comment=0017
You might not like him after it but it might be out sometime around next year. Until then he might get buffed he might get nerfed back down to put him off for a while.
Lissandra unfortunately is not in her best and hasn't been this whole season which says that devs are not really that much interested in her currently - well devs said it actually ;0. So you might want to wait for a few patches into the next season after announced changes to ap items (might come a new item or a few changes) to see how that will shape her together with rune adjustments.
For now I'd say Azir and Ekko are both nice options. With Azir you never know if devs are going to hammer him for a whole year again or not. So far they posted they are satisfied with how he turned out so far. But we are yet to see a few more tournaments which will say if someone will exploit him and put him on a radar.
So my vote is either Ekko or Azir, both fun. I play both and have no problem with either. Ekko being a safe assassin which is nice. Azir with a very unique style and very strong is very impactful is a good option too (I love Azir ;0). Both can make interesting plays so pick whichever suits your style more.