My favourite grunts have to be the SS guards who go 'Mein Laben!' when they die in Wolfenstein 3D (and their Doom 2 counterparts).
Then again...they are technically gearing more towards advanced/elite troops than 'grunts' in which case I would also have to say that the yellow raptor guys from Turok 2 were also favored (the way they run away from my Cerebral Bore gave me a warm feeling inside).
Then again...they are technically gearing more towards advanced/elite troops than 'grunts' in which case I would also have to say that the yellow raptor guys from Turok 2 were also favored (the way they run away from my Cerebral Bore gave me a warm feeling inside).