Mariners #3 choice: Zack Wheeler
Since it's highly unlikely the Mariners would have taken anyone actually drafted later than the 1st round in 2009, I'm selecting from the 49 1st round picks including the supplemental picks. Of course, the Giants drafted Zack Wheeler at #6, four picks after Ackley. He ended up with the Mets via the Carlos Beltran deal, where Wheeler put up a 3.77 ERA and a 100 ERA + over five seasons. He only had the five years because he underwent TJ surgery in March 2015, but he managed to start at least 29 games in three separate seasons for the Mets.
Would the Mariners have extended Wheeler and benefitted from the late blooming that the Phillies have enjoyed? Maybe, maybe not. It's difficult to ascertain if Wheeler would have been better or worse in the Mariners organization, so perhaps we should assume neutral development.
I have Wheeler at #3 instead of #2 largely because his elite performance, both in the regular season and postseason, has come after his free agency. His six years of team control under the Mets were solid, but they weren't elite and he wasn't considered a frontline starter. His tour in Philly has been a different story, a 51-28 record with a 2.95 ERA, what could a guy like that have meant for these late Mariner clubs?