Oil is back below $30 -- and that could be great news for investors in oil stocks. Three oil stocks in particular: Weatherford International (NYSE: WFT), Halliburton (HAL -0.42%), and Schlumberger (SLB 0.34%).

In a rare confluence of events, all three of these oil-service stock majors have received new buy ratings this morning. Even better, the stock market is ignoring the ratings. With oil prices once again in decline, all three stocks are following the oil market lower:

  • Schlumberger -- down 0.9%,
  • Halliburton -- down 1.6%, and
  • Weatherford -- down an astounding 6.5%!

So why is this good news? According to analysts at DA Davidson, so long as you can stomach some volatility in the oil market, all three of these stocks are 100% buyable today. And according to our data on Motley Fool CAPS, Davidson is one of the better analysts we track. It ranks in the top 15% of investors.

All well and good. So Davidson has shown its stuff, and proven it usually knows what it's doing. But of the three stocks it's recommending today, which one should you buy first?

Weatherford International
Let's begin with the worst prospect of the three. With $2 billion in accumulated losses over the past year, Weatherford is not any kind of obvious bargain. Indeed, according to data from S&P Global Market Intelligence, Weatherford hasn't earned a profit for four years straight.

That said, through drastic cuts in capital spending, Weatherford finally lifted itself into positive free cash flow territory last year, generating a modest $24 million in cash profit -- the first time it had accomplished such a feat in five years.

According to Davidson, this $6 stock is worth about $9. True, the stock carries substantial debt -- $7 billion net of cash. But despite what investors may fear, Davidson sees no risk of bankruptcy at Weatherford, because the company has "ample headroom" on its debt covenants, and the risk of it defaulting on a loan is not "realistic."

The situation at Halliburton is even more optimistic. Despite boasting a market capitalization several times larger than Weatherford's, Halliburton carries a smaller debt load -- barely $5 billion net of cash.

Halliburton isn't profitable, granted. But prior to 2015, it posted several years in a row of profitable operations, and its free cash flow record is superb. Last year, the company generated positive free cash flow of $722 million -- and judging from analyst estimates, Halliburton has every chance of growing that number in the years ahead. Long-term growth rates are expected to average 13.5%, and the company's planned merger with Baker Hughes (NYSE: BHI) -- Davidson also rates Baker Hughes a buy -- remains on track.

But wait -- we've saved the best for last: Schlumberger. As Davidson opines, "Large cap companies have always taken share in the downturn ... SLB is the largest oilfield service company in the world and should disproportionately outperform in this environment due to its scale, global diversification, technology leadership, and game-changing Cameron acquisition."

Schlumberger isn't an obvious buy at 44 times earnings, but the mere fact that Schlumberger is the only stock on Davidson's buy list that is earning profits at all is already an accomplishment. It puts Schlumberger head and shoulders above the rest. Last year, Schlumberger reported earnings of $2.1 billion, and its free cash flows are even more prodigious -- $5.2 billion.

Weighed against a $90.6 billion market cap that's more than three times the size of Halliburton, and with a net debt load not much bigger than Hally's, Schlumberger stock costs only 17.4 times free cash flow today. With a 10% projected growth rate and a 2.8% dividend yield, that's starting to look attractive -- and could grow more so if oil prices rise, and Schlumberger's growth rate rises with them.

Of the three stocks winning buy ratings from Davidson today, Schlumberger is the stock I like best.