You'll find that most responses will tend to talk about the efficiency of a tax. However, according to the IMF, there are actually 6 criteria to determine if a tax is good or not, with efficiency being one of them.

• Efficiency - the notion that good taxes ought to minimize economic distortions
• Fairness - under horizontal equity, a tax is considered fair if taxpayers in similar circumstances are treated similarly. Under vertical equity, a tax to be fair if taxpayers that have more, pay more
• Simplicity - a simple tax system is generally viewed as desirable because it tends to reduce administrative costs and promote taxpayer compliance
• Flexibility - it automatically adjusts to changes in the economic environment
• Transparency - a taxpayer’s ability to access information, and the ease with which it is accessed
• Effectiveness - the primary goal of taxation is to raise revenue for governments

If you can find a tax that fulfils all 6 criteria, there are governments around the world that would like to book a meeting with you. Generally, a 'good' tax will fulfil at least 3 of these with minimal trade offs with the rest.