My criteria: A functional 25 man roster of Americans to comprise the best team possible. I can pick injured players, working under the assumption I simply get them at the ability they'll be at when they come back (i.e. I could include a guy currently out with Tommy John surgery, but I run the inherent risks that come with it). This team is limited to America-raised American citizens who have not played for other countries in international competitions (so, for instance, the American-born Adrian Gonzalez is not eligible for the team since he played for Mexico in the World Baseball Classic). I don't feel great about my bullpen.
Starting Lineup
C--Buster Posey
1B--Paul Goldschmidt
2B--Jason Kipnis
SS--Troy Tulowitzki
3B--Josh Donaldson
LF--Bryce Harper
CF--Mike Trout
RF--Giancarlo Stanton
A few of these guys I think are indisputable--Posey, Goldschmidt (another guy is probably close, but trails Goldy by pretty much any measure, if not by an enormous amount), Donaldson, Trout, and Stanton (unless you're much more scared of his current injury than I am, I guess). Jason Kipnis was unexceptional in 2014 but he was good in 2013 and is actually the ZiPS projected best second baseman in baseball for the rest of the season, so I'll take the moderate risk. Tulowitzki has been mediocre in 2015 thusfar but I was so confident in his stature as baseball's best shortstop before this season that I'm standing by him. And as for Harper, I tend to be bullish on the notion that this is actually what Bryce Harper is now. Maybe not THIS, but something close enough to it to make him a slam-dunk candidate for this lineup.
The Bench
C--Jonathan Lucroy
1B--Anthony Rizzo
3B/SS--Manny Machado
3B/2B--Anthony Rendon
CF--Andrew McCutchen
Lucroy has had a disturbingly down 2015, but he is a year removed from being better than Buster Posey for a full season, and there isn't really another great option as far as American catchers go (since Puerto Rico is a separate nation from the United States for international sports purposes, their catchers are not included as eligible for Team USA, to my eternal annoyance). Rizzo is a slightly poorer man's version of Goldschmidt and would be an incredible lefty option off the bench as a pinch hitter. Machado might be the second best defensive shortstop in baseball behind Andrelton Simmons, although he is instead relegated to being the best defensive third baseman in baseball, and he has seemingly turned the corner from being a fine MLB hitter to being a really, really good one. Rendon and Matt Carpenter got some consideration for the same bench role since they have experience at both second and third base in the Majors but I went with Rendon for three reasons--he's a righty whereas Jason Kipnis (of whom I'm more skeptical than Josh Donaldson) is a lefty so there's a possible platoon there, he has balanced the positions more recently (Carpenter hasn't played second base since 2013), and I'm scared of Matt Carpenter's recent tailspin. I don't expect a lot of disputes of Cutch except for those who would like a fifth outfielder, but with the ones I have, I can't imagine ever wanting to actually start #5.
The Rotation
Clayton Kershaw
Max Scherzer
Chris Sale
David Price
Corey Kluber
Scherzer has been so dominant after his move to the NL that there's a case for him as #1, but it's hard to go against Kershaw's track record. Sale and Price are high-end pitchers among each of the three measures I examined by and large: 2014 production, 2015 production, and 2015 projections for the rest of the season. As for Kluber, the fact that he would never be included in discussion for this team a year ago does scare me a bit, but he's been so good for the last season and a half that I'm not THAT scared.
The Bullpen
Wade Davis
Dellin Betances
Jake McGee
Madison Bumgarner
Zack Greinke
Chris Archer
Jacob de Grom
I went with three true relievers and yet I didn't pick a true closer. But I feel all right about these guys. Wade Davis has been a dominant reliever since he moved from starting, Betances seems to be held back from closing mostly for silly Yankees ideas about what a "closer" is, and McGee has been the most dominant non-Chapman lefty reliever in baseball for the last year and a half. Bumgarner, annoying as his hype is, is a rock solid MLB ace or near-ace, depending on how picky you are. Greinke gets overshadowed by Kershaw but there isn't a better #2 in baseball going right now. Archer has been one of the three or four most impressive pitchers in baseball this year, and although he wasn't QUITE this guy last year, there's enough track record to make me believe this is at least mostly not a fluke. As for de Grom, I'm sure Matt Harvey would be a popular pick among the Mets young starters, but the one I'm most confident will be a really good starter (if not a Cy Young winner) for the next decade is Jacob de Grom. So I went with this. I still feel like my bullpen is missing somebody.