[R] selecting rows by maximum value of one variables in dataframe nested by another Variable

[Miriam]

How could I select the rows of a dataset that have the maximum value in one variable and to do this nested in another variable. It is a dataframe in long format with repeated measures per subject.  
I was not successful using aggregate, because one of the columns has character values (and/or possibly because of another reason).
I would like to transfer something like this: 
subject    time.ms  V3 
1		1	stringA
1		12	stringB
1		22  	stringC
2		1 	stringB
2		14	stringC
2		25	stringA
…. 
To something like this: 
subject	   time.ms	V3
1		22	stringC
2		25 	stringA
… 

Thank you very much for you help!
Miriam
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