My own choice is Phil Kearny.
Buford was replaced by other able Union cavalrymen like Merritt, Sheridan etc.
Lyon was replaced by the competent Samuel R. Curtis.
McPherson's death didn't really have much of an impact on the course of the Atlanta Campaign.
Mansfield was elderly and probably wouldn't have risen any higher than corps command.
Reynolds was replaced by other able Union corps commanders like Hancock, Parke etc.
Sedgwick died late in the war when the outcome had pretty much been decided.
Smith was an able officer, but Grant pretty much covered his loss.
Sumner was old and being shipped off to the Trans-Mississippi when he died.
Wallace was replaced by competent Western officers.
That leaves Kearny. His death in 1862 deprived the Union command in the East of a highly aggressive and competent general. The Eastern theatre is where the Union really lacked this kind of commander up until 1863/64. He might have risen to corps/army command and been more of a challenge for Lee than Burnside, Hooker et al.