The other person who you replied to makes good points - it’s always going to be context dependent and it drives me mad when I’m out and about and have so many signals projecting “leave me alone” (such as wearing headphones, being on the phone, studying etc.) and a guy hits on me.

However, if someone is generally approachable, I’ve found that the best compliments are on something the person has consciously chosen about their appearance. So stuff like graphic t-shirts (especially band t shirts), hair styles (I love people with dyed hair because this presents to me an easy option for compliments).