(Composite of photos by Bloomberg)

For most workers, an employer-provided 401(k) is the primary retirement savings vehicle.

But how easy the plans are to use and how generous they are can vary widely depending on where you work, according to a Bloomberg study released Tuesday. Near the top of the list are ConocoPhillips, Boston Scientific and Visa, which offer generous employer matches and vest employees in the savings immediately. But Facebook, Whole Foods and Costco were near the bottom of the list.

Bloomberg ranked 250 large employers' 401(k) plans based on  the company match, investment options offered, how long employees had to wait before they would be vested and whether companies enrolled people automatically. The analysis was based on 2012 data from the Department of Labor. (Ten of the companies' plans could not be ranked.)

Top-ranked 401(k) plans according to Bloomberg

The most generous company, the Bloomberg study found, was ConocoPhillips, which matched 9 percent of employees' annual salaries, even for workers who contributed as little as 1 percent of their income, for example. It was followed by Philip Morris, which provided a 5 percent match for employees' retirement accounts.

Among the least generous employers were familiar names like Costco, Whole Foods and Wynn Resorts, which contribute set dollar amounts into 401(k) plans every year instead of matching a percentage of employees' contributions. Costco and Wynn gave $500 each, and Whole Foods contributed $152 a year as of 2012, according to the report.

Wynn told Bloomberg it has since increased its match to $750. Costco and Whole Foods pointed out they offer other generous perks, including health benefits for part-time workers in Costco's case and low health insurance premiums for people who work at Whole Foods.

Dead last was Facebook, which didn't offer a 401(k) match in 2012, the latest year for which data was available. The company started offering a match in April, according to Bloomberg.

Bottom-ranked 401(k) plans according to Bloomberg

This report comes amid a growing debate over what should be done to get people to save more for retirement. Having access to a retirement savings plan at work is usually viewed as the first step. Indeed, studies suggest people are more likely to save if the process is made seamless through payroll deduction and even more so if they are automatically enrolled into the plans.

What's more, pensions have largely been replaced by 401(k)s, putting more of a burden on workers to save for their later years. Still, many people, especially those working part-time, don't have access to the accounts through their jobs, prompting some lawmakers and financial pros to call for the creation of a national or state-run savings program.

Separate studies show some people may want more from their workplace retirement plans. A survey released in May by Towers Watson, a professional services consulting firm, found that less than 50 percent of workers felt their plans would meet their needs in retirement. Most employees said they would give up more pay if it meant they could have guaranteed retirement benefits.