Please Help Me Choose

Greetings,

I have played every warhammer 40,000 army that i can think of( except sisters of battle and all kroot army). Well I could never choose so i quit warhammer for a year. Well I got back into it and I bought a Night Lords army. I played em and like them. Well then i bought a huge IG cadian army. Well over the last month ( thats when i started again) i took a look at my armies and got very upset. I didnt do a good job putting the figs together or cleaning them or anything so now i am going to sell me armies and start from sratch and I am going to put them together correctly, model them WYSIWYG, and clean them properly. Now my big debate is which army should i get. I thought about what armies i enjoyed the most and narrowed it down to them. So its IG, Necrons, Night Lords or Kroot( now i have never played kroot but i like the list and idea of the army). SO can you please give me your opinion on which of those armies i should collect and please tell me why you think i should.

Thank you for your precious time ( i am not being sarcastic)