Best practice for engineer's performance appraisal

I know there are a lot of TAC friends in this community, I was wondering if somebody can share some good ideas for doing a fair performance appraisal for TAC engineers.

I noticed in many situations that the focus is to take in consideration the quantity rather than quality of work. The classic individual ranking, which adds up a bunch of metrics like number of cases, average handling time and others, It will only favour the guys who are hunting the easy cases and It will ruin the team spirit.

I'm wondering if somebody can share some good examples of fair performance appraisal.

Thank you!