2019 is on pace to be another great year for investors. All three major U.S. stock indices are up by double-digit percentages since January, which is impressive given that market returns have been strong for more than a decade.

In times like these, true bargains are few and far between. Yet we Fools know that there are always great stocks to buy. Which ones in particular? We asked a team of Motley Fool contributors to highlight their top stock picks right now. Their choices: American Tower (NYSE:AMT)Enterprise Products Partners (NYSE:EPD), Etsy (NASDAQ:ETSY), Apple (NASDAQ:AAPL), and Livongo Health (NASDAQ:LVGO)

STacks of dollar bills next to sand timer

Image source: Getty Images.

It's rarely a bad time to buy this specialty REIT

Tyler Crowe (American Tower): You may not have heard of American Tower, but chances are, you've indirectly paid them without even knowing it. This specialty real estate investment trust owns and rents out space on cellphone towers to major telecommunications companies that need to expand their networks' coverage. It's perhaps one of the most boring businesses out there, but it makes up for it with a bulletproof business plan, great returns on investment, and monumental growth opportunities still ahead of it.

The beauty of American Tower's business model is its simplicity. Its clients benefit because it spares each of them from having to build entire towers just for their own equipment, which isn't very cost-effective. American Tower, on the other hand, can lease space on all of its towers to multiple telecom companies simultaneously, and generate far better returns.

According to management, the return on capital invested for a tower with one tenant is 3%; raise that to three tenants and the return jumps to 24%. What's more, since it doesn't own any of the telecommunication equipment, its maintenance expenditures on those towers are incredibly low, meaning that the vast majority of that cash can be invested in growth projects or paid out as dividends to investors

For a real estate investment trust, American Tower's dividend is nothing to write home about -- it yields 1.7% at the time of this writing. It makes up for it, though, with its good record of growth, and plenty of potential to grow in the future. Each time a telecom company increases the speed it offers to the users of its network, it needs to increase the amount of network equipment that moves that data. So as mature economies deploy 5G networks, and emerging economies continue to add to their 4G infrastructure at rapid rates, there will be plenty of opportunities both in the U.S. and abroad to sign up new tenants and acquire new towers.

Shares of American Tower rarely trade at cheap valuations because it is a stellar and durable business. However, if you're looking for a long-term investment, this REIT checks all the boxes. 

This top-tier, high-yield energy stock is on sale

Matt DiLallo (Enterprise Products Partners): Enterprise Products Partners is one of the largest midstream companies in the energy sector. Because of that, it generates lots of steady cash flow. It has already hauled in nearly $5 billion in cash through the first nine months of this year, which is 14% more than its total through the same period of 2018. It distributed about 60% of those funds to its investors via a dividend that currently yields an attractive 6.9%, and reinvested the rest into expanding its operations.

Enterprise Products currently has $9.1 billion of growth projects under construction, which is one of the largest backlogs in the sector. Projects underway include expansions of several of its pipelines, building new export facilities, and constructing another petrochemical plant. These projects should be coming online through 2023, which gives investors a fair amount of visibility into its growth prospects. Those projects should also provide Enterprise with the fuel to continue boosting its payouts, which it has done for the last 22 consecutive years.

Despite the MLP's success in 2019, its shares have gone on sale over the past few months. The unit price was recently about 15% below its high for the year, and down about 4% from where it sat 12 months ago. Because of that, this top-tier energy stock now has one of the lowest valuations in the sector. Between the cheap price, the above-average yield, and the highly visible growth prospects, Enterprise Products Partners is one of the best stocks to buy this December.

Handmade profits

Brian Feroldi (Etsy): There are a lot of reasons for investors to like Etsy. The e-commerce platform connects millions of buyers with sellers of unique, handcrafted goods that cannot be found anywhere else. This provides the company with a defensible market niche. And that market is probably bigger than you think: Etsy estimates it accounts for $100 billion in annual spending today.

Etsy already controls about 4% of this market, which is a big enough slice to allow the company to generate profits and free cash flow. But management believes that's the company's growth is just getting started. By expanding geographically and adding additional categories, Etsy estimates that its total addressable market opportunity could grow to nearly $250 billion.

The company's recent results show that it's making the most of that enormous opportunity. Its numbers of active buyers and sellers grew 21% and 27%, respectively, while revenue jumped 32%. Management is actively investing in the business, so net income took a step backward, but the company remains solidly profitable. 

Despite all that, Wall Street wasn't impressed with the results and investors sold off Etsy's stock. That has left shares trading near a 52-week low. While the shares still don't look classically cheap, I think that the company's strong growth is here to stay, and that investors who buy now will be well-positioned to earn multibagger returns over the next decade.

There's still plenty of new opportunities for this seasoned tech stock

Chris Neiger (Apple): Some investors may look at Apple and think only of how iPhone sales are declining. But there's far more going on with Apple than just its flagship smartphone, and overlooking its opportunities in both wearable technology and services would be a huge mistake.

Apple's biggest opportunity for growth comes from its services segment, which includes everything from the company's App Store and Apple Pay, to its Apple Music, Apple News+, and newly launched Apple TV+ subscription services. Services revenues grew by 18% in the most recent quarter to a record $12.5 billion, and the segment now provides 19% of Apple's total revenue (up from 16% last year).

That's all good news, and there's likely more where that came from. The launch of Apple TV+ only came earlier this month, and a potential new bundle of services is reportedly in the works. In short, at this point, Apple has only begun to exploit the potential of its services opportunities.

But that's not all. Apple also has one of the most successful lineups of wearable technology, with its Apple Watch, AirPods, and Beats headphones. In the most recent quarter, revenue from Apple's wearables, home, and accessories segment grew by 54% year over year. The company is the leader in the segment and research shows that consumers' wearable tech spending is poised to jump 55% between now and 2021.  

While Apple may not be able to rely on the iPhone for revenue growth anymore, the company's impressive gains in both its wearable technology and services segments prove that there's still a lot to love about this tech stock. 

This personalized healthcare company is changing the game for diabetics

Sean Williams (Livongo Health): In 2017, the Centers for Disease Control and Prevention released its National Diabetes Statistics Report. Contained in that report was an estimate that, as of 2015, 30.3 million people were living with diabetes (about a quarter of them undiagnosed), and another 84.1 million were exhibiting signs of prediabetes.

Left untreated, prediabetes often leads to type 2 diabetes within five years. That meant more than 114 million people were at risk in 2015 -- and the figure has likely since gone higher since then. That means there's an enormous opportunity for Livongo Health to revolutionize how diabetics (and prediabetics) take care of themselves. 

What Livongo provides are solutions that not only help diabetics better understand the metrics that should matter to them, such as their blood glucose levels, but incite actual behavioral changes in patients with chronic conditions. Arguably the biggest barrier to treating diabetics is the diabetics themselves, who may lack the motivation or solutions to change their eating habits, testing regimens, or their insulin injections.

Livongo corrects these behaviors more successfully than traditional diabetes solutions through its wirelessly connected ecosystem. By collecting more than 1 million aggregate blood glucose readings from its members every few weeks, and having its researchers analyze this data, as well as other behavioral factors from its members, Livongo is able to provide actionable tips that encourage diabetics to take action and live healthier lives. 

How's it working? Over the past year, Livongo Health's member count has more than doubled from 95,000 to almost 208,000, with sales skyrocketing 148% to $46.7 million in the third quarter from the prior-year period. Most notably, non-GAAP gross margin is already a healthy 75%. The business model is built on subscriptions, which should lead to highly predictable revenue and cash flow, and, as noted with the diabetes figures earlier, it has an exceptionally long growth runway. 

Considering that Livongo Health is expanding its ecosystem to treat prediabetes, hypertension, and weight management as well, this high-growth company just might be the stock to buy in December.