Source: Newfield Exploration Co.

Oil and gas prices have been incredibly volatile for the past year, which has led to just as much, if not more, volatility among energy stocks. Some have been absolutely crushed by weak commodity prices as their stocks are touching all-time lows. Others, however, are having a much easier time of things and are off to a really good year. Here are three of the best-performing energy stocks of 2015:

Tyler Crowe: Before you get too excited to see that Williams Companies (NYSE:WMB) stock has gained a little more than 25% this year, let's remember why that happened. It's not from some spectacular results, but rather it's the result of the company receiving an offer from Energy Transfer Equity to acquire the company for $64 per share in June.

Don't get me wrong: There are a lot of things that Williams is doing right. It has an extremely robust backlog of development projects that has the potential to significantly grow its payout to shareholders by 10%-15% over the next five years. However, this is something that the company has had on the books for a couple years now, so it's really hard to say that this 25% jump is from investors suddenly realizing that this is a great growth plan.

Considering that so much of the company's stock performance has been attributed to this one-time speculative deal, it's hard to say if the company itself can actually live up to those expectations. Management has also asked for offers from other companies to see if it can rebuff Energy Transfer on its unsolicited offer. Considering the share price premium that Energy Transfer is offering, though, it's hard to see Williams management not taking the deal after testing the waters. 

Bob Ciura: At first glance, one might naturally assume that a more than 50% drop in oil prices would hurt oil refiners, but that hasn't been the case. Stock prices of many refiners are up over the past year, including that of Valero Energy (NYSE:VLO), whose shares are up 31% just since the start of 2015.

The reason? Refiners actually benefit from lower oil prices. As oil prices fall, so do refining feedstock costs. This causes refining margins to widen, and helps boost oil refiners' profits. For example, Valero's earnings rose 40% last year to $6.97 per share. Then, Valero's earnings grew another 21% in the first quarter, year over year, due to the benefit of an increase in refining throughput volumes.

As its fundamentals improve, Valero is aggressively returning cash to its shareholders. Valero's most recent $0.40-per-share quarterly dividend was 60% higher than the same distribution last year. Plus, Valero is using a significant portion of its profits to buy back stock. The company bought back 5.4 million of its own shares for $325 million in the first quarter.

Going forward, Valero has significant strategic initiatives planned to continue growing. The construction of Valero's two crude topping units at the Corpus Christi and Houston refineries is progressing and on track. Upon completion, these facilities should further reduce feedstock costs at both of these refineries.

In contrast to many of its peers in the oil and gas space that are struggling to stay afloat, Valero is thriving. 

Matt DiLallo: Newfield Exploration (NYSE:NFX) takes the cake as the best-performing energy stock this year among S&P 500 companies. Through the first six months of the year, its stock was up nearly 36%, easily outperforming the basically flat index. 

Newfield's strong 2015 showing was initially fueled by the company's solid fourth-quarter 2014 report at the end of February. That report, as well as the company's outlook to boost production 8% in 2015 while cutting capex by 40%, sent shares soaring by double digits. It then took advantage of its higher stock price to raise $815 million of equity capital, which it subsequently used to repay its credit facility. On the heels of its equity offering, it also announced a debt offering whereby it raised $700 million. That cash was used to pay off higher yielding debt that matured sooner. These were moves that really bolstered its balance sheet and liquidity, giving it plenty of financial flexibility to weather the weakness of commodity prices. 

Aside from the increased financial flexibility, the other big driver for Newfield Exploration in 2015 has been its premier position in the promising STACK and SCOOP plays of the Anadarko basin. The play is turning out to be a significant growth engine for the company as production is expected to be up 45% year over year. Meanwhile, well costs are coming down, which are improving Newfield's returns. 

Newfield's combination of balance sheet strength and a returns-driven growth engine has earned it a lot of praise from analysts this year. Its stock has been upgraded to the buy list by several notable analysts and these upgrades have attracted new buyers. Those buyers have fueled a rising stock price, which is why Newfield Exploration is the best-performing energy stock through the first half of 2015.