You can find plenty of employers that offer retirement plans with generous matching contributions and low-cost investing options. But if you want access to the 10 best 401(k) plans, it helps to be an airline pilot, a quarterback, a roughneck or a chemist.
Nearly 50,000 retirement plans, covering more than 57 million workers and about $4 trillion in assets are ranked each year by Financial research firm BrightScope. The best 401(k) plans have many common features: They have generous matching contributions and offer a large menu of low-cost index funds and set-it-and-forget-it target-date funds.
Here are the top 10 401(k) plans, according to BrightScope:
1. Delta Air Lines
Assets in plan: $7.53 billion
Plan features: The Delta Pilots Savings Plan charges less than 0.25 percent per year and the average participant has a $504,000 account balance.
2. National Football League
Assets in plan: $1.78 billion
Plan features: The NFL Player Second Career Savings Plan has a profit-sharing component and more than 8,200 active participants.
3. Saudi Aramco
Assets in plan: $1.95 billion
Plan features: The 4,400 participants in the Saudi Arabian Oil Company savings plan have an average account balance of $460,000.
4. Southwest Airlines
Assets in plan: $3.20 billion
Plan features: Fifteen percent of the assets in the Southwest Airlines pilots' 401(k) plan are invested in Vanguard index funds.
Assets in plan: $6.88 billion
Plan features: Nearly 20 percent of the assets in ConocoPhilips' 401(k) plan are held in company stock, which can be a risky investment for workers, and 10 percent of assets are in target-date funds.
Assets in plan: $3.99 billion
Plan features: Twenty-two percent of the assets in Amgen's 401(k) plan are invested in company stock and 18 percent are held in a Northern Trust S&P 500 index fund.
Assets in plan: $4.26 billion
Plan features: Bayer's 401(k) plan moved up 12 places in BrightScope's rankings with more than $3,700 in company contributions per participant and more than $3,000 in salary deferrals per participant.
Assets in plan: $17.09 billion
Plan features: A whopping 41 percent of the assets in Chevron's 401(k) plan is held in company stock and 14 percent is invested in Vanguard stock and bond index funds. (Only 7 percent of all 401(k) plan assets were invested in company stock as of 2014, the most recent data available, according to the Employee Benefits Research Institute.)
9. Takeda Pharmaceuticals
Assets in plan: $1.48 billion
Plan features: Nearly 30 percent of the assets in Takeda's 401(k) plan are held in a Northern Trust S&P 500 index fund.
Assets in plan: $1.29 billion
Plan features: The UPS/IPA Defined Contribution Money Purchase Pension Plan has 21 percent of its assets in Fidelity stock and bond index funds.
BrightScope scored the plans on more than 200 data points found in public documents that large employers must file with the Labor Department and Securities and Exchange Commission. Some of the most recently available public data was current as of Dec. 31, 2015.
Retirement plan sponsors continue to shift assets from actively managed mutual funds to lower-cost index funds that track market benchmarks. Index funds represented nearly 41 percent of assets help in the 30 top 401(k) plans, a 6 percent rise since BrightScope's last analysis.
More than 10 percent of the assets held in BrightScope's top 30 401(k) plans are invested in target-date funds, which are pre-mixed portfolios that automatically invest more conservatively over time.
Don't fret if you're not lucky enough to work for a company with a highly rated 401(k) plan. "These 401(k) rankings are the tip of the iceberg," said Trisha Brambley, CEO of Retirement Playbook, a retirement plan advisor search firm.
Most retirement plan sponsors will continue to focus on lowering costs for participants, Brambley said. "Every little bit helps increase retirement savings," she said.
This article originally appeared on CNBC. Read more from CNBC: