Workers and retirees looking for a health plan with the best dental coverage have their work cut out for them during the federal health insurance open enrollment season that ends Dec. 10.

Most are eligible to belong to any of about two dozen plans. They range from traditional fee-for-service plans to union-backed plans and health maintenance organizations. All of the plans in the federal program are good. But benefits and premiums vary, and smart shoppers can save thousands of dollars next year in premiums and out-of-pocket costs by picking the best plan for them.

Under the federal health program, workers and retirees can switch plans each year during the open season. No employee or retiree can be refused coverage because of age, health or preexisting medical conditions. If a worker or employee exhausts the lifetime benefit in one plan, he or she can switch to another during the next open season and start all over again. Retirees get the same coverage and pay the same premiums as workers. The government pays about 60 percent of the total of the average premium.

When it comes to dental benefits, the experts say, the health maintenance organizations win hands down. In his "Checkbook's Guide To Health Insurance Plans" for U.S. workers, Walton Francis says all the plans with the "best overall dental coverage" are HMOs.

If dental benefits are your chief concern, he recommends that you read the brochures of Aetna, George Washington University and HealthPlus high option plans, and those offered by Capital Care and Potomac HMO.

His insurance rating guide (available from Checkbook at 806 15th St. NW, Suite 925, Washington 20005) says that the above-mentioned HMOs don't have deductibles or maximum dental benefit limits. They cover preventive care in full, and reimburse for common and inexpensive procedures such as fillings. He says they cover about two-thirds of the more expensive procedures.

If you prefer a fee-for-service plan, Francis says that the following plans cover about half of all dental costs. They include BACE, Blue Cross standard option for families with children, MailHandlers high option, NAPUS, National Treasury Employees Union, Postmasters high option and Rural Letter Carriers plan. BACE is limited to congressional and legislative branch employees while the Rural Carriers plan is limited to association members.

Plans that will pay for about one-quarter of dental costs include Alliance high option, American Postal Workers Union, Blue Cross standard (for adults), Foreign Service, GEHA, Letter Carriers and Postmasters standard. Francis says none of those plans should be the first choice of a family expecting big dental bills next year. Executive Pay Raises

In yesterday's item about 1991 executive pay, I dropped a line that further confused the already complicated pay issue. Members of the Senior Executive Service (with six pay levels) now have a pay range from $71,200 to $83,600. Next month the president is expected to issue an executive order that will move SES pay rates in January to a minimum of $87,000 and a maximum of $108,300.