Time magazine declares that "70 is the new 50." And just because you're retired doesn't mean it's time to stop investing. In fact, retirement may be the best time to invest in stocks. Seriously, when have you ever had so much free time to do the things you want to as you do in retirement? No longer chained to the office, and freed from the time commitments of commuting to and from work, retirement finally gives you the downtime you need to really get to know the stocks in your portfolio -- and even research some new ones.

With that in mind, we asked three of our top Foolish investors to each suggest just one stock that they think worthy of consideration as an investment. In response, we got Dominion Energy (NYSE:D), Celgene Corporation (NASDAQ:CELG), and Deutsche Bank (NYSE:DB). Now here's why.

An old man in a ball cap sits in a blue folding chair beside a tree-lined body of water, with a fishing pole cast into the water.

Will the fish bite today? And will these stocks go up? Image source: Getty Images.

There's tremendous growth potential in this utility

Neha Chamaria (Dominion Energy): I'm sure you aren't surprised to see a utility on this list. The defensive nature of the power and gas business, after all, makes utilities great investment options for retirees. Choices are aplenty, but the one stock that has caught my attention is Dominion Energy, thanks to its solid growth catalysts.

Dominion operates an expansive network of electricity and natural gas transmission, storage, and distribution that serves nearly 6 million customers. The company has its eyes on the next growth level as it brings its ongoing megaprojects online, the most significant being its Cove Point liquefaction facility that will liquefy natural gas and export it as liquefied natural gas. The project is on track to go online this year, and Dominion has already signed 20-year export contracts with an international energy marketing company each from India and Japan. Simply put, Dominion has already locked in a significant portion of sales for the project.

That aside, Dominion has several other projects in progress, all of which are expected to boost the company's cash flows substantially in the coming years. Dominion already has some financial goals in place: It is targeting compounded annual average earnings-per-share growth of 6%-8% through 2020 and "at least" 5% beyond that. Better yet, management aims to grow its annual dividends by 10% over the next three years.

In short, from high earnings and dividend growth potential to a handsome current yield of around 4%, Dominion offers everything a retiree would want from a stock.

Ignore the critics

George Budwell (Celgene Corp.): Biotech heavyweight Celgene has come under fire from a handful of analysts lately because of the potential entrance of generic competition for its flagship cancer medicine, Revlimid, far earlier than expected. Although there is a remote possibility that generic versions of this megablockbuster blood cancer drug will indeed hit the market before its patents expire in 2026, Celgene has so many other value drivers outside of Revlimid that this particular risk factor is arguably overblown at this stage.

Admittedly, Revlimid does make up the lion's share of Celgene's revenue stream, but that situation is changing quickly. The biotech's fairly new psoriasis drug, Otezla, has already attained blockbuster status and is on track to eventually generate peak sales in excess of $2 billion within a few short years. Celgene's experimental multiple sclerosis drug, ozanimod, is also firing on all cylinders in its late-stage program. That's key, because ozanimod has the potential to become Celgene's next flagship drug, with peak sales in the neighborhood of $4 billion to $6 billion -- depending on how many indications it garners in total.

Further down the line, Celgene is also working toward becoming a leader in the high-value adoptive cell therapy space through its collaborations with bluebird bio and Juno Therapeutics, among others. Celgene's sizable external pipeline of adoptive cell therapy candidates could produce several billion in additional sales for the biotech within the next three to four years.

The bottom line is that Celgene's risk from generic competition is a real issue that needs to be monitored -- but its immense pipeline of novel medicines should keep its top line headed in the right direction regardless of how this patent drama plays out. That's why this top biotech is arguably a great buy for even the most conservative of investors.    

It's always darkest before the dawn -- in Germany

Rich Smith (Deutsche Bank): If you ask me, Deutsche Bank offers an incredible bargain for investors with the patience to profit from it -- and a high tolerance for negative headlines.

Deutsche Bank doesn't look like much at first glance. In recent years, it's seen its profits sapped by multiple legal entanglements -- mostly tied to the financial crisis, but also involving shenanigans and market manipulations. As a result, Deutsche Bank stock is currently trading at "record lows" (says the Financial Times), underperforming most of its peers, and valued much more cheaply than most U.S. banks as well.

But things often look darkest before the dawn. So what does the bright side look like? Valued at just 0.43 times book value, Deutsche Bank is one of the cheapest big banks on the planet right now. Its stock trades for less than half the price-to-book ratio of Citigroup or Bank of America, for example, and less than a third the ratio of JPMorgan Chase. Yet this is the fourth biggest bank in Europe we're talking about here, and the biggest bank in Europe's fastest-growing country,  Germany.

Simply put, as bad as things look for Deutsche Bank in the rearview mirror, I can't help thinking there's nowhere to go but up. Analysts who follow Deutsche's fortunes expect the bank to re-emerge into profitability next year -- with a P/E ratio of less than 10, which is cheaper than any similarly sized U.S. bank's stock. Long-term predictions call for respectable 7% annualized growth over the next five years, and the stock pays a modest 1.3% dividend .

If you're retired and you have the time, I think this is a bank stock worth watching.

George Budwell has no position in any of the stocks mentioned. Neha Chamaria and Rich Smith don't, either. The Motley Fool, however, owns shares of and recommends Celgene, and The Motley Fool recommends Dominion Resources. The Motley Fool has a disclosure policy.