Actually, Patrick Corbin is the best available free agent in baseball

Since the start of the 2018 MLB season, Bryce Harper and Manny Machado have dominated every discussion of this offseason's free-agent market. 
But, despite what you may have heard for the last year (or more), neither Harper nor Machado sits at the top of a sensible ranking of free agents. That spot belongs to D-backs left-handed starter Patrick Corbin, who is coming off an All-Star season in which he threw 200 innings with a 3.15 ERA.  
Here's why:
Young, reliable starters are rarely available
It's really hard for a team to find a top-shelf starting pitcher if it doesn't already have one in its system. They're rarely available through trade either, as most teams with a frontline starter are going to be in the mix for a postseason spot. When they are available, a team has to empty its farm system to get him.
Then there's free agency, but young No. 1 starters don't really pass through there either. Both of last offseason's top starters (Yu Darvish and Jake Arrieta) had already pitched their age-30 seasons. The previous year's market was headlined by Jeremy Hellickson and Ivan Nova, who no one was mistaking for aces. You have to go back to the 2015-16 offseason to find anyone like Corbin on the market, when Zack Grienke, David Price and Johnny Cueto headlined the class. However, all were older than Corbin. 
You just don't see pitchers like Corbin on the free-agent market very often. That scarcity makes him incredibly valuable.
And, there are tons of good shortstops and outfielders
Recent years have seen impact outfielders like J.D. Martinez, Lorenzo Cain, Jason Heyward and Justin Upton hit free agency. Moreover, recent trades have seen star outfielders like Christian Yelich, Giancarlo Stanton and Tommy Pham change teams. This year, a team can walk away with Michael Brantley, A.J. Pollock or Harper.
At shortstop, Machado was the prize of the Trade Deadline less than four months ago. There are just so many shortstops around baseball who can both field and hit that there is bound to be another one available during the season.
In other words, recent history has shown that if you want an elite shortstop or outfielder, an opportunity to acquire one will arise outside of free agency. There's just not the same urgency to get one now as for a pitcher.
His repertoire is well-suited to the current environment
Pitchers are increasingly relying on the breaking ball to get outs in a strikeout-heavy environment. Over the past three seasons, Corbin has increased the use of his slider to the point that he threw it 40.9 percent of the time in 2018, according to Statcast. Against that slider, opponents whiffed on 53.5 percent of their swings, and that pitch alone generated 195 of Corbin's 246 strikeouts in 2018.

In an age where players are hitting the ball harder than ever before, Corbin has countered by allowing fewer and fewer balls in play, increasing his strikeout rate from 18.7 percent in 2016 to 30.8 percent in 2018.
The rise of bullpenning makes elite starters more valuable
Sure, starters may be throwing fewer innings than ever before and teams appear to be increasingly comfortable instituting bullpen days as a matter of course during the regular season. But, that makes starters who can pitch six or more innings even more important to prevent relievers from becoming overtaxed. Twenty four of Corbin's 33 starts last season lasted at least six innings, taking the load off the bullpen.
Days off for relievers are more scarce than ever, which makes starters who can pitch deep into games more valuable than ever. That's good news for Corbin.