Sarcastic answer: His favorite son-in-law is Mr. Wickham.
Actual answer: it is probably a bit of a tie between Mr. Bingley and Mr. Darcy (assuming Kitty and Mary either have no husbands or do not marry so well).
How can I say this? Well, lets first go to the text with regards to his sentiments about Mr. Wickham.
"I admire all my three sons-in-law highly," said he. "Wickham, perhaps,
is my favourite; but I think I shall like _your_ [Elizabeth's] husband quite as well
Now, if you just read the above without ever having read the rest of the novel, you would assume that Mr. Bennet like Mr. Wickham the most. Natural assumption to make.
However, I would like to conjecture this is not the case at all. Mr. Bennet is notorious through the novel of being ironic, sardonic, satirical, and generally a pretty bad narrator of his own story. So to read the text above and to understand what he truly means are completely separate.
Here's where we need to really delve into the realm of interpretation.
It is my personal belief that Mr. Bennet would probably like Mr. Bingley most. For starters, Mr. Bingley quite enjoys his routine. Mr. Bennet's library at Longbourn, in Pride and Prejudice, is a zone of mystery and, eventually, of guilt. We know from the book that it is a ground-floor room and that it contains a writing table, at least two chairs, and a quantity of books. Mr. Bennet habitually goes there after breakfast and stays most of the day, coming out for dinner and tea but going back between tea and supper while the rest of the family might more sociably be reading aloud, playing backgammon, or having some music. "[W]ith a book," we are told, "he was regardless of time". He likes to have the room "to himself", but on several occasions his solitude is breached, notably, of course, when he has to entertain marriage proposals for his daughters.
From this routine we can hazard a guess that any disruption from said daily goings-on in the world of Mr. Bennet would result in a reduced opinion on behalf of Mr. Bennet.
Lizzy marrying Darcy? Well that in itself is not particularly disruptive. A bit of a hooplah at the start but eventually the excitement would die down. But! his wife would be too intimidated by Mr. Darcy himself to cause too much of a ruckus directly to Elizabeth or Darcy. Who bears the brunt of her "nerve"? Mr. Bennet (and potentially Mary if she remains at home a spinster).
Not the case for Bingley and Jane. Bingley is so open and welcoming and just so... nice that if Mrs. Bennet has any dramatics about anything - which she always does - she will and most assuredly does take it all out on the couple directly. Which is probably why, after only one year of marriage, the Bingley's move closer to Pemberley.
Of course Mr. Bennet would get hit with some shrapnel or fallout from a Mrs. Bennet blow-out, but not nearly as bad as how I imagine it would play out with Mrs. Bennet having a tiff with Elizabeth and Darcy.
So for me, Mr. Bennet's favorite son-in-law is the son-in-law who provides an outlet for Mrs. Bennet to release her nerves upon outside of himself.
Mr. Bennet's favorite son-in-law is Mr. Bingley in my humble (and I truly mean that) opinion.
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