Once when I was in New York with my family, and looking somewhat disastrous (if you've walked around New York in August, you know what I am talking about), a guy stopped dead in his tracks, looked me down, and then back up, and said "Hot DAAAAMN!".
Not followed by a pick-up line, not accompanied by a leer, just straight-up appreciation. I felt amazing for days after that. So with that in mind, if you think someone is awesome, tell them! Friend, partner, family, stranger... sharing your admiration is free, and it can make someone's day.