Which one should I get?

Is 'Rekt' a good answer? xD jk jk.

I personally like supporting/tanking more so I voted Tahm Kench, but if you main mid you should learn Azir.

Are you a mid lane or a versatile lane? Technically Azir can be played in other lanes, but the learning curve should be less on TK.

I love azir and his playstyle but he has one hell of a learning curve esp as his method of approach changes quite a bit depending on the champion.

Tahm is the fun pick (bruiser tahm top is incredible).

I'd go for azir because he's different and a good way to improve skillful play.