Source: Flickr user fsecart.

When it comes to your retirement, it's practically impossible to set your portfolio up for long term success without adding some high-quality dividend stocks. A single-digit dividend yield may not seem like much on a year-to-year basis, but reinvested and compounded over many decades, it could mean the difference between simply retiring and retiring the way you've always dreamed of.

Yet not all dividends are equal. Some dividend payments are simply so low they aren't even worth your time, while some companies' business models or payout ratios may suggest that the dividend could be unsustainable. Thus investors are always searching for stocks that deliver high dividend yields and can also maintain and/or grow those payouts over the long run.

Overall, 2014 has been a pretty good year for high-yield dividend stocks (I've arbitrarily defined a "high-yield dividend stock" as any stock with a yield of 5% or more). Of the 316 high-yield stocks that have a market value over $300 million, 116 have risen by a minimum of 10% as of Monday's market close. Dividend stocks can sometimes take a backseat to a strong bull market run as investors chase riskier growth stocks, so this is an encouraging sign that the appetite for regular income remains strong.

But high-yield dividends also come with their own risks. In some instances, the only reason a yield is high is because a company's share price has fallen dramatically or the dividend is unsustainable. Either case could potentially lead to a dividend cut, which can subsequently hurt the underlying share price of a stock.

With these benefits and risks in mind, let's have a look at the five best-performing high-yield dividend stocks of 2014 not only to examine why they're doing so well, but also to analyze whether their high payout is sustainable.

1. New Media Investment Group (NYSE:NEWM): 87% year-to-date return
The surprising top-performer for the year thus far is print and online media assets company New Media Investment, which went public earlier this year. On the surface, you have to scratch your head and wonder how a company focused on local newspapers and publications is excelling. But dig a bit deeper into the mobile and website asset business, and that's where the bulk of New Media Investment's future growth lies.

The upside of a company like this is that it plans to continue acquiring local assets in order to drive growth. The more assets it acquires, the more diversified its business will become, which should protect investors against major swings in its cash flow. The downside, however, is that these cash raises are often coming in the form of stock offerings that can be dilutive to investors.

All told, its projected 5.7% yield looks steady for now, considering its near-doubling in same-basis year-over-year cash flow, but at 25 times forward earnings, it doesn't promise much in the way of value.

Source: Kool Cats Photography via Flickr.

2. Emerge Energy Services (NYSE:EMES)85% year-to-date return
Emerge Energy Services would be the hands-down best high-yield dividend stock of 2014 if we had looked at these high-yield dividend stocks in September. Shares of Emerge are down nearly 50% since then as falling oil prices have sapped expectations for the company's fuel terminal operations. Additionally, the company's sand operations used in fracking could see weakness if oil and gas companies decide to cut back on production due to weaker fossil fuel prices.

Despite the clear commodity weakness, Emergy Energy's third-quarter results were strong, with its declared distribution increasing 18% from the sequential second quarter. Given that its business is dependent on drilling rig growth, I would expect its current 7.1% yield to fluctuate a bit in the coming quarters, but I also believe a reasonably high yield in excess of 5% is likely sustainable.

3. Frontier Communications (NASDAQ:FTR)55% year-to-date return
Landline and broadband provider Frontier has been a battleground stock among bulls and bears for as long as I can remember. Following two dividend cuts in recent years, skeptics have every reason to believe another dividend cut could be in the offing, considering that Frontier's 2014 payout is more than double its expected EPS.

On one hand, Frontier keeps adding to its broadband customer base and has slowly built up nearly 400,000 video subscribers. The advantage of its customer platform is that it generates predictable cash flow, and the company maintains reasonably strong pricing power because there are few communications companies for rural consumers to choose from.

Yet Frontier has been losing landline customers at a steady clip since it completed a deal to buy rural landline assets from Verizon in 2009. So long as Frontier continues to battle high debt levels and declining landline subscribers, it's likely to remain a subpar high-yield dividend stock.

Source: Flickr user James Jordan.

4. NorthStar Realty Finance (NYSE:NRF)50% year-to-date return
One way to secure yourself a nice payout is to invest in real-estate investment trusts, or REITs, which are required to pay out 90% or more of their earnings in order to receive an advantageous tax status in the U.S.

NorthStar Realty Finance is a commercial REIT that's taken advantage of low lending rates in order to build up its portfolio of assets. It has agreed to acquire Griffin-American Healthcare REIT, a medical office building and senior housing REIT in the U.S. and U.K., for $4 billion, and it is also acquiring a handful of hotel portfolios. Though I would suggest investors keep a close eye on NorthStar's leverage to ensure it doesn't overextend itself, so long as the U.S. economy remains strong, NorthStar's 8.7% yield will probably remain steady.

5. BGC Partners (NASDAQ:BGCP)48% year-to-date return
Rounding out the five best high-yield dividend stocks of 2014 is wholesale financial and commercial real estate brokerage BGC Partners, with a 48% year-to-date gain.

Like many of the other high-yield companies mentioned above, BGC is benefiting from a strong U.S. economy, which has boosted its real estate service operations. Furthermore, the return of market volatility resulted in a 74% increase in pretax distributable earnings in the third quarter. In fact, BGC's third-quarter post-tax earnings were a record for the company.

Also fueling excitement around BGC is the company's tender offer for GFI Group, which should help boost the diversity of its financial operations while simultaneously strengthening its electronics business. If this deal does go through, then BGC's forward P/E of 12 and its yield of 5.6% might prove to be a bargain.