With Jose Quintana off to the Cubs, Oakland's Sonny Gray is pretty clearly the best available controllable starting-pitching option on the trade market. "Best" may be actually underselling it, because the starting market is pretty barren. Gerrit Cole? The Pirates entered Friday only three games out of first place in the National League Central. Justin Verlander? He's got two more seasons at $28 million per, plus a full no-trade clause. Yu Darvish? Perhaps, but he's a rental. So is Lance Lynn.
Given the market forces at play -- along with Billy Beane's recent comments about committing to a full rebuild, and the fact that Gray has considerably rebuilt his value with some outstanding recent starts after a poor and injury-plagued 2016 -- it seems like a near certainty that a move is looming, perhaps before his next scheduled start on Tuesday in Toronto. 
So who needs Gray the most? Where would he fit the best? Let's break down some options.
Briefly, what you need to know about Gray: He'll be 28 in November, he's making $3.6 million this year and he's under team control for two more seasons past this one. Though Gray's 3.66 ERA doesn't stand out, he's allowed only six earned runs over his past five starts, and the most advanced Statcast™ metrics have him performing like Giovany Gonzalez, Marcus Stroman or Darvish this year.

The all-too-obvious fit

Even before the season began, it was assumed the Astros would make a move to reinforce their rotation, and now that they're clearly the best team in the American League -- and have had that rotation hit hard by injuries -- that hasn't changed. Houston has the need, either providing insurance for Dallas Keuchel and Lance McCullers against October health issues or creating a formidable top three. The Astros have the talent, with six highly-rated prospects on the MLBPipeline Top 100.
It's such an obvious fit that perhaps it's too obvious -- and it feels like those deals never actually end up happening.
The big names who may have already made their big move

 Though the Yankees have added Tommy Kahnle and Player Page for David Robertson to Albertin Chapman, Dellin Betances, Chad Green and Adam Warren in what's suddenly become a stud-filled bullpen, they still have huge rotation issues. While Luis Severino looks great, Michael Pineda is out for the next year after undergoing elbow surgery, Jordan Montgomery may be running out of gas (4.60 ERA past eight starts) and Masahiro Tanaka has been consistently inconsistent. But they already paid a high price to get Kahnle, Robertson and Todd Frazier, and it's hard to see them going further to trade Gleyber Torres or Clint Frazier as well.

 It's something of the same story for the Cubs, who, despite adding Quintana, could use another good starter -- not just for now, but also the future, since Jacob Arrieta and John Lackey are set to be free agents and there's no starter coming from the farm system. With only one remaining prospect on the Top 100 (Jeimer Candelario), the Cubs' once-vaunted prospect pipeline is thinner than it was. They may be a better fit for Darvish, since he'd be less costly as a rental.

The pennant winner with an unexpected need

 Cleveland hasn't yet been able to pull away from Kansas City or Minnesota, and despite how loaded its rotation looked entering the year, it may actually need some reinforcements. Sure, Corey Kluber and Carlos Carrasco look great, but Josh Tomlin, Trevor Bauer and Danny Salazar all have ERAs north of 5.00. (Plus, Salazar has been out for over six weeks with a shoulder issue, and he was sent to the bullpen before that anyway.)
While Mike Clevinger has stepped up -- he's basically pitched like Gray this year -- Cleveland certainly remembers what it was like to be short staffed in the rotation last October. Besides, Gray's salary surely fits in its budget for the next two years.
The surprising sleepers

 Though the Brewers have lost five in a row, allowing the Cubs to slice the NL Central margin to a single game entering Friday, they've come so far this year that they have to be in the "buyer" category, but they need to do it smartly. An expensive rental doesn't seem like a fit, but a cost-effective starter who can not only help them this year, but in 2018 and '19? That makes sense. Put Gray with Jimmy Nelson and Chase Anderson, each of whom made big steps forward this year, and suddenly the Brewers have an intimidating trio -- and they definitely have the prospects to do it.

 It's somewhat similar in Atlanta, where the surprising Braves have played near-.500 ball. They aren't contenders this year, but they hope to change that soon, and their 2018 rotation remains unsettled, with Julio Teheran, Mike Foltynewicz and Sean Newcomb the likely first three, and plenty of questions beyond that. We heard all the rumors about Atlanta having tried for Chris Sale; consider this along the same lines. No need to wait for the offseason to start building next year's roster.