It is better not to do that. (often preferred) It is better to not do that. (this is also fine) It's best if I don't do that. It would have been better had I not done that. It is better that the work was not done It would have been better had the the work not been done

-It is better to do not do that. (No) -It is better to not do that. (okay) -It was better to not do that (okay) -It is better, if I do "not" do that (Okay, as long as you use "not" instead of "no") -It was better, if I did not do that (No) -It became better, if I did not do that. (No) -It is better (that) the work is not done (you can probably say this, but I’m not sure what you trying to convey.) -It was better (that) the work was not done (same comment as above) -The undone work is better (No, this does not make sense)

It is better to not do that. The rest are gibberish. Please study sentence construction.