This is the question you have been waiting for

[Joles]

So while I am a bit positively biased towards C/C as a pax, I often struggle with a genuine compliment to give them at the end of an exceptionally well handled flight. So that is all I say.

Are there more words or phrases that can help them feel they have REALLY nailed it ? Genuine admiration ?

[Stanwell]

OK.
Upon disembarking, I'll generally pause, smile, look them in the eye and say sincerely...
"Thank you ... Well done."
They get the message.

[Croissant2016]

"Really nice flight; thanks!"
I think it's less on what you say and more on the emotion you out with it. Anyone can say thankyou but the people who say thanks, smile and look really pleased to be saying thanks convey their appreciation.

[crewmeal]

A few weeks ago I congratulated an Egyptair crew on their excellent service provided. They couldn't believe it saying no one ever says kind things to us anymore.

[what next]

It's not just cabin crew who deserve a compliment every now and then! Recently I flew as passenger and the check-in counters were seriously understaffed, who knows why. Long queues, nervous passengers, an extremely unpleasant atmosphere. When it was my turn to drop my luggage, the lady had just picked up the phone to call her supervisor: "My shift is over now, but it's totally crazy out here, never seen such a queue. Is it OK if I stay an hour longer and help to resolve this mess?" After she had labelled my suitcase, I said to her: "Wow! What an attitude you have towards your job. I guess not many of your colleagues would have done that." She looked at me totally incredulous and and said: "Thank you, that the first time anyone has ever said anything nice to me since I am on this job."

[Pequena_Inquieta]

I think the best thing you can do is ask to speak to the supervisor/purser and tell them you'd like them to send a compliment to the crew's manager. I've sent compliments about my colleagues and luckily I've received compliments from my own manager who happened to be on my flight. That could help them with a prospective upgrade.
But the little compliments on disembarking also made my day (in the past, because I'm now resigning). Comments such as "Thank you for the lovely flight", or "Thank you for the amazing service" along with a genuine smile etc. They're always welcome.
Another thing really touched me was when pax showed genuine concerns towards my wellbeing, such as asking after the service if I had eaten already. Or if I had had my rest. Little things, great impact. :)

[Joles]

It's not just cabin crew who deserve a compliment every now and then! Recently I flew as passenger and the check-in counters were seriously understaffed, who knows why. Long queues, nervous passengers, an extremely unpleasant atmosphere. When it was my turn to drop my luggage, the lady had just picked up the phone to call her supervisor: "My shift is over now, but it's totally crazy out here, never seen such a queue. Is it OK if I stay an hour longer and help to resolve this mess?" After she had labelled my suitcase, I said to her: "Wow! What an attitude you have towards your job. I guess not many of your colleagues would have done that." She looked at me totally incredulous and and said: "Thank you, that the first time anyone has ever said anything nice to me since I am on this job."
This has been useful ! Airline crews both on the ground and C/C often go through thankless hours trying to keep things going and by a strange Murphy's Law on exactly those days encounter difficult pax ! I would quit if I were in their place. So to me they appear very very well -deserving of a genuine compliment. Which is why the post. Thanks for all the responses .More views are welcome too

[Evanelpus]

With over five and a half thousand views and only six comments I would beg to differ with the OP