The link for this might seem odd, but in my experience the best way to do this is to fill their contact form: https://www.klm.de/en/claim

Choose the compliment/suggestion option. I do this regularly for flight crews on AFKL, or even ground staff, when they go beyond the normal level. I also know that this method actually works because I've had crew confirm to me that this ends up in their files, helps with promotions, and I've had occasionally crew thank me for the comments I sent in (in case I encounter them again).