I have been told that 'you look good' is the perfect compliment to give. Because 'you are looking good' is not colloquial.
In order for the phrase “You are looking good” to be really colloquial, the subject and the auxiliary should be contracted: You're looking good is the form preferred, and native speakers will contract words, especially auxiliaries, whenever possible.
“You look good” cannot be contracted in writing, but in speech the words you and look will slide and merge until you get the following result:
Y'look (/jə'lʊk/) good
I'm not 100 % sure I transcribed that in eye-dialect or IPA correctly, so please edit.
Both exclamations, or pleasantries if you prefer, are grammatically acceptable because it's how the majority of English speakers express themselves today.
The OP has also been told that the perfect compliment is the latter but I'm not sure I would agree, the following praise:
You look stunning/amazing/gorgeous/hot tonight
is the perfect compliment for any woman :).
Good is a fairly bland, and nondescript adjective in my "book". Nevertheless, both ‘You look good’ and ‘You're looking good’ share the same advantage, they have no social or gender barriers, they can be said to both men and women, adults and children, friends and distant family members alike.
To me, you're looking good sounds more emphatic, and more enthusiastic than you look good, and used when a person you haven't seen in a while, has improved their physical appearance.
On the other hand, ‘you look good’ is often used in conjunction with the verb make when a person needs to create a positive and favorable impression of themselves; e.g.
Notice that while you're complimenting others, you're not supposed to be self-deprecating. You don't want to make yourself look bad, you want others to look good. [...] "But what if my boss is a jerk?" you ask. It's still in your best interest to make him or her look good. Believe it or not, it will make you look good. — Career Coach
It's my job to make you look good, and it's your job to make the company look good.” “Then we make a great team. With you taking care of my wardrobe, I should be vice president in no time.” “I hope so.” — Does She Love You?
He is your employee, who needs your help to become a good employee and make you look good. It's pretty simple. To be a good manager, you need to understand management rights and responsibilities, create good standards for your ... — Management Experience Acquired
In none of the examples I cited above, does look good mean to be physically attractive.