Словосочетание At best переводится на русский как «в лучшем случае». Объяcнение словаря: at best used to emphasize that something is not very good, pleasant, honest etc even if you consider it in the best possible way. Соответственно at worst означает «в худшем случае». Словарь говорит: at (the) worst if things are as bad as they can be. Достаточно полезные выражения, на мой взгляд.
Вот примеры:

• Many drivers feel their job is unpleasant at best, and dangerous at worst.
• Choosing the right software can be time-consuming at best and confusing or frustrating at worst.
• The campaign was at best only partially successful.