Should I buy Aatrox, Ekko, or Taliyah?

Aatrox has been left in pretty trash tier by riot. So the other two depends on which playstyle you prefer, since they're both AP and build similar items currently... kinda.

IMO taliyah is so fun to play, but with the latest nerfs on PBE she will suck (even more than she does now after changing her W) also I think you should buy the champion you want with IP and get a skin for them better than wasting RP on 'em.

I find both ekko and taliyah incredibly fun to play, but they're not for everyone and take a bit to play well. I wouldn't buy either with RP though, I generally save rp for skins since champions can be bought with ip.