Mike Trout's Key Stats: 1,199 G, 5,273 PA, .305 AVG, .419 OBP, .581 SLG, 176 OPS+, 285 HR, 200 SB
WAR (Pos. Rank): 72.5 (1st)
Mookie Betts' Key Stats: 794 G, 3,629 PA, .301 AVG, .374 OBP, .519 SLG, 134 OPS+, 139 HR, 126 SB
WAR (Pos. Rank): 42.0 (2nd)
Giancarlo Stanton's Key Stats: 1,162 G, 4,897 PA, .268 AVG, .358 OBP, .547 SLG, 144 OPS+, 308 HR
WAR (Pos. Rank): 39.9 (4th)
Mike Trout is Mike Trout. No further explanation needed. Mookie Betts had the best single-season WAR (10.9) of any player during the decade when he won AL MVP in 2018, and his total WAR trailed only Trout among outfielders. Plus, his 113 DRS made him one of just four outfielders to eclipse the 100 mark in the 2010s. Another no-brainer.
The tough decision was Giancarlo Stanton vs. Andrew McCutchen. Both players won an MVP award and posted nearly identical WAR totals. The nod ultimately went to Stanton, thanks to his sizable edge in OPS+ (144 to 135).
It would feel weird to discuss the top outfielders of the 2010s without at least name-dropping Bryce Harper. From the moment he was chosen No. 1 overall in the 2010 draft, there was perhaps no baseball player who was more frequently in the spotlight. And in 2015, at least, he was the best of them all.
Likewise, McCutchen and Jose Bautista made their marks on the decade. The former by being a five-tool wonder between 2011 and 2015. The latter by launching 227 homers between 2010 and 2015, as well as popularizing the "Elevate and Celebrate" style of hitting that's all over baseball today.