A teacher recently asked me, "What should I praise, achievement or effort?" Realistically, you should praise both, but in appropriate ways. For example, if you only praise those students who score at a certain point, some students never receive positive attention for their work. It's important to praise and reinforce quality work, but we do need to consider those who do their best, and still don't "make an A".
I balanced my praise, making sure I always commented on effort, regardless of the grade (even those who score high grades need to be reminded they had to work to make that happen). Robert Marzano, a well-known researcher, points out that effort can be increased in students if we reinforce it in positive ways. In other words, students pay attention to what we pay attention to. What are you reinforcing with your students?