Well Black War Greymon is definitely a strong digimon, but not even he can defeat Grimmjow. Grimmjow is a supremely powerful espada who just can’t lose. Grimmjow has his powerful cero which would probably take care of Black War Greymon. Grimmjow is too strong. Grimmjow wins.
DReager1.com 
Inaccurate. Grimmjow got destroyed by Ichigo’s Vizard form who in turn got killed by Ulquiorra’s second release form. Ulquiorra’s second release is only city level and hypersonic in speed, making Grimmjow town level. Black Wargreymon is FTL in both speed and reactions, capable of stomping skilled digimon and has planet level attack potency. The current version of Grimmjow in the newly released chapters got knocked out by a quincy’s ball of poison. He’s weaksauce. If anything Black Wargreymon should be a high above not Grimmjow who only has a losing streak for his fights.
Black War Greymon may technically be FTl due to the nature of Digimon and how they’re digital, but his combat speed is nowhere near FTL and I wouldn’t even call it hypersonic. I consider Grimmjow to be a lot faster than any Digimon and that’s the big turning point in this round.