The short answer
It depends on your risk. Many guidelines aim for under 100 mg/dL for most adults, under 70 for high risk, and under 55 for people with heart disease or very high risk.
For LDL, lower is better, and there's no level found to be too low in trials. Your clinician sets your goal from your overall risk.
Source: 2018 AHA/ACC cholesterol guideline; 2019 ESC/EAS dyslipidaemia guideline.