I usually save compliments for the end of the tanda, so it doesn't distract from enjoying the dance together (I've noticed if someone compliments me right after the first song, I sometimes dance the second poorly because I'm trying to repeat the element they complimented a minute ago instead of just dancing). But something short and sweet, like "that was lovely" usually works for me.
I would only say something if I have a positive comment to make, never complement if it's not deserved.
Also, I would never pass a judgement like "you are a good dancer" or similar. Rather I would express how I felt dancing with her.
To a beginner: "You've been dancing tango already, haven't you?"
The best I've come up with, that is, an original compliment of mine, not something I had read somewhere:
"Usually I'm happy with this time change, coz'it makes the week-end last longer. Yet this year it made me sad, as it delayed
for one hour the opportunity to dance with you."
(We have time changes twice a year)
Argentine men are so good at giving compliments. We foreign women are sitting ducks when given some of these - they are so rare here. Example: "I must go to the emergency room - your beautiful dance is too much for my heart"