There are a number of ways to exclaim about body part pairs, which vary in their level of politeness/formality/respect.
"Wow! She's got nice eyes/legs!"
is fairly respectful, but not formal, though this is more to do with the enthusiasm inherent in it being an exclamation, and depends on the body part to which you refer. "She's got beautiful eyes/a beautiful smile" is quite neutral.
"She's got a nice pair of legs!"
could be used, but it is not particularly polite or complimentary ("She's got a good set of legs!" is something you might say about a work horse, for example). You would not typically hear "pair of eyes" here - this just sounds clinical.
"(She's got a) nice pair!"
is rather vulgar plus typically means breasts.
"Look at those legs/eyes!"
(sexy/nice/whatever are implied).
"Look at the legs on her!"
is not very respectful, but is more so than "Look at the legs on that one!" One would probably not use "eyes" here.
"Whoa! Sexy/Nice legs/eyes!"
Not so respectful, as it is rather blunt, but is something you might say to a friend who's just got dressed up/made up (like, "Whoa! Nice haircut!")
Note: "Wow, what sexy legs/eyes she has!" is perfectly fine, but is generally not used because it echoes Little Red Riding Hood observing the wolf dressed as Grandma: "Grandma! What big ears you have!" ("All the better to hear you with, my dear.")