In a few days Mr. Bingley returned Mr. Bennet's visit, and sat about ten minutes with him in his library. He had entertained hopes of being admitted to a sight of the young ladies, of whose beauty he had heard much; but he saw only the father. (Chapter 3)

I agree, where Mr. Collins heard of the Bennet sister’s beauty is a puzzler. Mr. Bingley also heard about the Bennet sister’s beauty before meeting them however I took the source of his intelligence to be the amiable Sir William who visited him early. Sir William also did Lizzy the compliment in Chapter 6 suggesting to Darcy, “You cannot refuse to dance, I am sure, when so much beauty is before you.” LoL, Sir William does not begrudge a compliment. Mr. Collins does not seem to know anyone in Meryton before his arrival however. ;D

He had not been long seated before he complimented Mrs. Bennet on having so fine a family of daughters; said he had heard much of their beauty, but that in this instance fame had fallen short of the truth; and added, that he did not doubt her seeing them all in due time well disposed of in marriage. This gallantry was not much to the taste of some of his hearers; but Mrs. Bennet, who quarrelled with no compliments, answered most readily -- (Chapter 13)

It could not be Darcy because Mr. Collins does not introduce himself to him until Chapter 18 and he has already heard about their beauty before he comes to Meryton. His first reference to the beauty of the Bennet daughters is in Chapter 13 soon after his arrival—“he had not been long seated…” I do not think his university or Hunsford acquaintance can be completely ruled out but I agree it does seem unlikely. His father seems an unlikely candidate also because when he writes to Mr. Bennet he references no prior personal relationship with him suggesting the feud between his father and Mr. Bennet has been in place a long while and saying of his father, it had “always pleased him to be at variance” with Mr. Bennet. ;D

She had always spoken to him as she would to any other gentleman; she made not the smallest objection to his joining in the society of the neighbourhood, nor to his leaving his parish occasionally for a week or two, to visit his relations. (Chapter 14)