My ideal company would be one that offers 6-figure salary, 5 day work week and unlimited vacation.
If this isn't realistic then I'd have to say these:
Reputation (stability and security of the company is important for me)
Career Advancement (opportunities - no explanation needed)
Work Life balance (this keeps me motivated. I believe it makes for a less stressed, happy employee)
This is just for me and it isn't the same as what others may want.
If this question is being asked to me in an interview then I will align my answer of an ideal company to the company I am applying to.