It all depends on your style of play
CSMs are rather expensive on points, so an army that's cheap on points such as Orks, Tyranids or best of all (IMO) Imperial Guard would be a fun option
On the other hand since the CSMs are point heavy, not to mention pretty tough maybe an army similar such as Necrons or Space Marines would be a good choice
Not to mention it depends a lot on your style of play, if you like your army to have incredible firepower Imperial Guard, Tau and Space Marines are good choices.
If you prefer an army that'll rush into combat Tyranids, Orks and to an extent Grey Knights are good.
And if you like armies with specialist abilities then Eldar or Dark Eldar are good choices
All in all I'd suggest one of three. Eldar, Imperial Guard or Tyranids
'Huddle close to your Emperor if he makes you feel safe. He cannot save you for only Chaos is eternal'