Despite the usual belief that Remus was the one who provided constructive criticism out of the four, he could get pretty brutal at times with the criticism. 

In the Marauders, Sirius was the one who appreciated fully and open-heartedly, James was the one who was a bit picky on what to appreciate and what not to, Peter was the one who gave general opinion so as not to hurt anyone's feelings. 

But it was Rem who gave the truest and clearest remarks of them all, not giving a crap about whether he would be hurting the other's feelings by saying so or not.