The Congressional Budget Office in the US published a report on tax expenditures in 2019. Their estimate is that tax expenditures totaled around $1.2 trillion in 2019 (5.8% of GDP) and accounted for roughly three-quarters of the total budgetary effects of all tax expenditures that year. The smallest of the tax expenditures is the state and local tax deduction ($22 billion); the largest are the exclusion and deferrals for the contributions and earnings associated with pensions and retirement savings accounts ($276 billion) and the exclusion for employment-based health insurance ($280 billion including the payroll tax expenditure).
You can read the full report here.